Old couple drinking last drops of sloe gin outdoors (4,3)
I believe the answer is:
open air
'outdoors' is the definition.
(synonyms)
'old couple drinking last drops of sloe gin' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'couple' becomes 'pair' (synonyms).
'drinking' means one lot of letters goes inside another (inserted letters are consumed or drunk).
'last drops of' indicates one should take the final letters.
The last letters of 'sloe gin' are 'en'.
'pair' going around 'en' is 'penair'.
'o'+'penair'='OPEN-AIR'
(Other definitions for open air that I've seen before include "definitely not in the concert hall" , "natural setting" , "Uncovered" , "kind of concert" , "theatre" .)