Is to prosecute in spring (5)
I believe the answer is:
issue
'spring' is the definition.
(thesaurus)
'is to prosecute' is the wordplay.
'to prosecute' becomes 'sue' (I've seen this in another clue).
'is'+'sue'='ISSUE'
'in' is the link.
(Other definitions for issue that I've seen before include "Publish; children" , "litter" , "Matter of contention" , "Susie (anag.)" , "Circulate, distribute or equip with" .)