Frenzied activity for an unusual sleuth (6)
I believe the answer is:
hustle
'frenzied activity' is the definition.
(I've seen this before)
'an unusual sleuth' is the wordplay.
'an unusual' is an anagram indicator.
'sleuth' with letters rearranged gives 'HUSTLE'.
'for' acts as a link.
(Other definitions for hustle that I've seen before include "Deal with hurriedly" , "Put pressure on - sleuth (anag)" , "Jostle, sell aggressively" , "Jostle or push together" , "Move furtively - earn forcefully" .)