It can be in a contract for many a purpose (6)
I believe the answer is:
clause
'it can be' is the definition.
The definition suggests a singular noun which matches the answer.
'contract for many a purpose' is the wordplay.
'contract' becomes 'c' (this could be a standard abbreviation I've not previously seen).
'for' is a charade indicator (letters next to each other).
'many' becomes 'l'.
'purpose' becomes 'use' (synonyms).
'c'+'l'+'a'+'use'='CLAUSE'
'in a' is the link.
Can you help me to learn more?
(Other definitions for clause that I've seen before include "Subordinate part of sentence" , "Separate item in contract" , "Subunit of a sentence" , "Joker, say" , "Proviso" .)