Definitive crooks a diver must avoid (3,5)
I believe the answer is:
the bends
'a diver must avoid' is the definition.
(a diver might get the bends)
'definitive crooks' is the wordplay.
'definitive' becomes 'the' ('the' is the definite article in English).
'crooks' becomes 'bends' ('bend' can be a synonym of 'crook').
'the'+'bends'='THE BENDS'
(Other definitions for the bends that I've seen before include "Problem for divers" , "on high mountain roads?" , "suffering when going under?" , "Sickness which drivers must watch out for" , "Decompression sickness (suffered by divers)" .)