Poet thus keeps mostly happy, animated within (6)
I believe the answer is:
sappho
'poet' is the definition.
(ancient Greek poet)
'keeps mostly happy animated within' is the wordplay.
'keeps' becomes 'so' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'mostly' means to remove the last letter.
'animated' indicates anagramming the letters.
'within' is an insertion indicator.
'happy' with its final letter taken away is 'happ'.
'happ' anagrammed gives 'apph'.
'so' placed around 'apph' is 'SAPPHO'.
'thus' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for sappho that I've seen before include "Sixth century BC Greek poetess" , "Greek lyric poetess" , "Pa hops around the ancient Greek poetess" , "Pa hops about Greek poet from Lesbos" , "Greek female poet" .)