Writing includes odd parts of main contract (7)
I believe the answer is:
promise
'contract' is the definition.
(thesaurus)
'writing includes odd parts of main' is the wordplay.
'writing' becomes 'prose' (ordinary writing as opposed to poetry).
'includes' is an insertion indicator.
'odd parts of' indicates alternate letters (letters in odd-numbered positions).
The alternating letters of 'main' are 'mi'.
'prose' going around 'mi' is 'PROMISE'.
(Other definitions for promise that I've seen before include "plight?" , "Make a commitment" , "Give one's word" , "Hope; commitment" , "Give an undertaking" .)