Detective, one on crook's case (4)
I believe the answer is:
dick
'detective' is the definition.
(informal term for a detective)
'one on crook's case' is the wordplay.
'one on' becomes 'di' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'case' says to hollow out the word (remove centre letters).
'crook' with its centre taken out is 'ck'.
'di'+'ck'='DICK'
Can you help me to learn more?
(Other definitions for dick that I've seen before
include
"See 20 Across"
,
"Detective (slang) - one of the 14 films"
,
"-- Turpin"
,
"10