Urge a lot of horses to be fastener? (5,4)
I believe the answer is:
press stud
'be fastener?' is the definition.
(I have seen 'Small fastener ' mean 'press stud' so perhaps 'fastener' could also mean 'press stud')
'urge a lot of horses' is the wordplay.
'urge' becomes 'press' (synonyms).
'a lot of horses' becomes 'stud' (I've seen this before).
'press'+'stud'='PRESS-STUD'
'to' is the link.
(Other definitions for press stud that I've seen before include "Snap fastener" , "It may secure clothing" , "Virile young editor, perhaps" , "catch!" , "one that can hold things together" .)