North-bound motorway service area for pigs? (4)
I believe the answer is:
farm
'area for pigs?' is the definition.
(where pigs are kept)
'north-bound motorway service' is the wordplay.
'north-bound' says the letters should be written backwards (in a down clue, letters go up or 'north').
'motorway' becomes 'M' (as in motorway names - M1, M4 etc).
'service' becomes 'RAF' (Royal Air Force).
'm'+'raf'='mraf'
'mraf' back-to-front is 'FARM'.
(Other definitions for farm that I've seen before include "Productive land" , "Country business" , "Agricultural unit" , "Grow crops or livestock" , "Agricultural building" .)