Yes there is a defence cap, but for bosses the cap is much higher than what the game tells you.
I would say the info provided in the defence tab is quite misleading.
My advice to new or returning players is to stack defence as much as possible until you feel tanky enough,
then focus on attack and other stats. (you don't do damage if you die in few hits, you know)
I don't have the exact number here but I would recommend at least stacking 17k defence on your cleric and try BT again.
You should notice a sizeable amount of damage reduction from the boss as you get more and more defence.
Here are some things you can do to get more defence:
1. Get defence starstones, with secondary stat def + X% if you can.
red ones don't have defence stat and green ones don't have secondary stat def% stat so keep that in mind.
You can farm orbs in lv58 dungeons (meta Bluemoon, La Catedral, etc) and lv63 diamonds in Sapphire Keep.
There are two elites in Black Hill that drop lv72 red diamond and blue diamond too.
2. Reset specialization. (cost some gold)
The left side gives you more healing (in addition def and damage reduction) even though the right side gives you attack.
I would recommend two options: 41+ points in left / 36~39 points in left and 36~39 points in right.
The first option gives you access to the specialization at the top (more HP, healing).
The second option gives you access to the DMG dealt/Haste buff, but you don't get benefit from one of two specializations at the top.
Again, put points in def, damage reduction, healing, etc.
3. Swap into lv73 gold set, as mentioned above, though I think using +20 lv73 blue set should give you enough defence if you do 1 and 2.
Also as a note, only breastplate enhancements gives you defence.
Hope this helps.