Many fish stick together (5)
I believe the answer is:
cling
'stick together' is the definition.
(thesaurus)
'many fish' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for l00).
'fish' becomes 'ling' (ling is a kind of fish**).
'c'+'ling'='CLING'
(Other definitions for cling that I've seen before include "Hold on as for dear life" , "Adhere to - ring" , "Hold tightly, become attached" , "Stick close (to surface)" , "Adhere, hold tight" .)