Historic assembly that gave thanks in triumph (5)
I believe the answer is:
witan
'historic assembly' is the definition.
'witan' can be an answer for 'assembly' (I have seen 'Assembly of old' mean 'witan' so perhaps 'assembly' could also mean 'witan'). I'm not certain of the 'historic' bit.
'gave thanks in triumph' is the wordplay.
'gave thanks' becomes 'ta' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'triumph' becomes 'win' (a win is a victory or triumph).
'ta' put inside 'win' is 'WITAN'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for witan that I've seen before include "Anglo-Saxon assembly" , "Anglo-Saxon parliament" , "Assembly of old" .)