A chap can be a catalyst (5)
I believe the answer is:
agent
'a catalyst' is the definition.
(thesaurus)
'a chap' is the wordplay.
'chap' becomes 'gent' (I've seen this in other clues**).
'a'+'gent'='AGENT'
'can be' acts as a link.
(Other definitions for agent that I've seen before include "Double-zero-seven, for example" , "One's representative" , "Spy; service provider" , "Author" , "Sales rep" .)