Tenuous source for Shakespeare's Shylock (7)
I believe the answer is:
slender
'tenuous' is the definition.
(synonyms)
'source for shakespeare's shylock' is the wordplay.
'source for' suggests taking the first letters (I've seen 'sources for' mean this (the first letters or sources of the words)).
'shylock' becomes 'lender' (Shylock is a moneylender in The Merchant of Venice).
The first letter of 'shakespeares' is 's'.
's'+'lender'='SLENDER'
(Other definitions for slender that I've seen before include "Slight in form" , "Spare" , "remote" , "Thin and graceful" , "with a narrow outlook" .)