Political extremist sheltering union leader? Something fishy here (5)
I believe the answer is:
trout
'something fishy here' is the definition.
(trout is a type of fish)
'political extremist sheltering union leader?' is the wordplay.
'political extremist' becomes 'trot' (short for Trotskyist or Trotskyite).
'sheltering' indicates putting letters inside.
'leader?' says to take the initial letters (I've seen 'leaders of' mean this).
The initial letter of 'union' is 'u'.
'trot' enclosing 'u' is 'TROUT'.
(Other definitions for trout that I've seen before include "Rainbow, say" , "Edible fish" , "Tutor (anag.)" , "Irritating old person" , "old bag" .)