A drink; for detectives, a half of beer (5)
I believe the answer is:
cider
'a drink' is the definition.
(cider is a kind of drink)
'detectives a half of beer' is the wordplay.
'detectives' becomes 'cid' (abbreviation for Criminal Investigation Department).
'a half of' indicates taking half.
'beer' halved 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" .)