Detective sergeant drinking beer in the Valleys (5)
I believe the answer is:
dales
'the valleys' is the definition.
(dale is a kind of valley)
'detective sergeant drinking beer' is the wordplay.
'detective sergeant' becomes 'DS' (abbreviation for detective sergeant).
'drinking' means one lot of letters goes inside another.
'beer' becomes 'ale' (type of beer).
'ds' going around 'ale' is 'DALES'.
'in' acts as a link.
(Other definitions for dales that I've seen before include "(North England) valleys" , "Leads into poetic valleys" , "Yorkshire valleys" , "Open river valleys in hilly areas" .)