A drink, eg before returning dinner signal (6)
I believe the answer is:
eggnog
'a drink' is the definition.
(eggnog is a kind of drink)
'eg before returning dinner signal' is the wordplay.
'before' says to put letters next to each other.
'returning' shows that the letters should be reversed in order.
'dinner signal' becomes 'gong' (I've seen this before).
'gong' in reverse letter order is 'gnog'.
'eg'+'gnog'='EGGNOG'
(Other definitions for eggnog that I've seen before include "Hot alcoholic drink" , "A punch for bedtime, perhaps" , "Fortified drink similar to a flip" , "Winter cocktail" , "Drink which includes hot beer" .)