Drink with a chap in the Royal Engineers (3,4)
I believe the answer is:
red wine
'drink' is the definition.
(red wine is a kind of drink)
'a chap in the royal engineers' is the wordplay.
'a chap' becomes 'edwin' ().
'in the' means one lot of letters goes inside another.
'royal engineers' becomes 'RE' (abbreviation).
'edwin' put inside 're' is 'RED WINE'.
'with' acts as a link.
(Other definitions for red wine that I've seen before include "Vin rouge, like claret" , "Tent, say" , "Burgundy perhaps" , "Such as Shiraz or Merlot" , "Merlot or Malbec, say" .)