Detectives tucked into excellent cold tart (6)
I believe the answer is:
acidic
'tart' is the definition.
(thesaurus)
'detectives tucked into excellent cold' is the wordplay.
'detectives' becomes 'cid'.
'tucked into' means one lot of letters goes inside another.
'excellent' becomes 'ai'.
'cold' becomes 'c' (eg on taps).
'cid' inserted into 'ai' is 'acidi'.
'acidi'+'c'='ACIDIC'
(Other definitions for acidic that I've seen before include "Sharp-tasting, sour" , "Cutting" , "Containing a sour-tasting substance" , "Vinegary" , "that leaves a bitter taste" .)