Policeman turned up caveman's fingers (6)
I believe the answer is:
digits
'fingers' is the definition.
(I know this)
'policeman turned up caveman's' is the wordplay.
'policeman' becomes 'di' (abbreviation for Detective Inspector).
'turned up' shows that the letters should be reversed in order.
'caveman' becomes 'stig' (I've seen this before).
'stig' reversed gives 'gits'.
'di'+'gits'='DIGITS'
(Other definitions for digits that I've seen before include "Numbers 0 to 9" , "One to nine" , "Fingers, toes and numbers" .)