Was busy making underpasses (7)
I believe the answer is:
subways
'underpasses' is the definition.
(I know this)
'was busy making' is the wordplay.
'making' indicates anagramming the letters.
'was'+'busy'='wasbusy'
'wasbusy' anagrammed gives 'SUBWAYS'.
(Other definitions for subways that I've seen before include "Pedestrian tunnels" , "transport systems" , "Underground passages" .)