Was busy developing systems underground (7)
I believe the answer is:
subways
'systems underground' is the definition.
(underground transport systems)
'was busy developing' is the wordplay.
'developing' is an anagram indicator (letters developed into a new form).
'was'+'busy'='wasbusy'
'wasbusy' with letters rearranged gives 'SUBWAYS'.
(Other definitions for subways that I've seen before include "Underground passages" , "Pedestrian tunnels" , "transport systems" .)