Cold mug filled with a chocolate-like substance (5)
I believe the answer is:
carob
'chocolate-like substance' is the definition.
(substance from the carob tree)
'cold mug filled with a' is the wordplay.
'cold' becomes 'c' (eg on taps).
'mug' becomes 'rob' (I've seen this in other clues).
'filled with' is an insertion indicator.
'c'+'rob'='crob'
'crob' going around 'a' is 'CAROB'.
(Other definitions for carob that I've seen before include "Saint John's bread" , "(Seeds yielding) powder used as a chocolate substitute" , "Tree yielding chocolate substitute" , "Arabian tree with brownish-purple pods" , "that one could use for chocolate" .)