Fool gets union to strike (5)
I believe the answer is:
clout
'strike' is the definition.
(I know that clout is a kind of strike)
'fool gets union' is the wordplay.
'fool' becomes 'clot' (I've seen this in other clues).
'gets' is an insertion indicator (gets can mean captures or absorbs).
'union' becomes 'U' (common abbreviation as in Trades Union Congress).
'clot' going around 'u' is 'CLOUT'.
'to' is the link.
(Other definitions for clout that I've seen before include "hit hard" , "Cuff" , "political muscle" , "Bash" , "Smack - influence" .)