Take on a party post, though not having the heart (5)
I believe the answer is:
adopt
'take on' is the definition.
(synonyms)
'a party post though not having the heart' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'though not having the heart' suggests removing the centre (the heart is the inside of the word).
'post' with its middle taken out is 'pt'.
'a'+'do'+'pt'='ADOPT'
(Other definitions for adopt that I've seen before include "Accept as one's own" , "Foster" , "Take on (another's child)" , "Choose as one's own" , "Acquire" .)