A drink; for detectives, a half of beer (5)
I believe the answer is:
cider
'a drink' is the definition.
(alcoholic drink)
'detectives a half of beer' is the wordplay.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'a half of' suggests halving the letters.
'beer' cut in half is 'er'.
'cid'+'er'='CIDER'
'for' acts as a link.
(Other definitions for cider that I've seen before include "Beverage from apple juice" , "Apple-based beverage" , "Alcoholic drink from fermented apple juice" , "Alcohol from apples" , "Drink made from apple juice" .)