Hunt for whimsical extremists (5)
I believe the answer is:
prowl
'hunt' is the definition.
(I've seen this before)
'for whimsical extremists' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'extremists' says to hollow out the word (remove centre letters) (letters on the extreme ends of the word).
'whimsical' with its middle removed is 'wl'.
'pro'+'wl'='PROWL'
(Other definitions for prowl that I've seen before include "Move in a predatory way" , "Move around restlessly" , "Move as predator" , "Move in a predatory manner" , "Walk around predatorily" .)