A drink with a light-hearted girl (5)
I believe the answer is:
glass
'a drink' is the definition.
(thesaurus)
'with a light-hearted girl' is the wordplay.
'with' says to put letters next to each other.
'hearted' indicates the central letters.
'girl' becomes 'lass' (lass is a kind of girl**).
The middle letter of 'light' is 'g'.
'g' put next to 'lass' is 'GLASS'.
(Other definitions for glass that I've seen before include "(Material used in) mirror" , "measure of wine" , "Calm lake may be compared to this" , "Greenhouse material" , "such as one might hold at party" .)