A drink with a light-hearted girl (5)
I believe the answer is:
glass
'a drink' is the definition.
(a glass of something)
'with a light-hearted girl' is the wordplay.
'with' means one lot of letters go next to another.
'hearted' indicates the central letters.
'girl' becomes 'lass' (lass can informally mean a girl).
The centre of 'light' is 'g'.
'g'+'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" .)