Warps back to large urban expansion (6)
I believe the answer is:
sprawl
'urban expansion' is the definition.
(I've seen this before)
'warps back to large' is the wordplay.
'back' says the letters should be written in reverse.
'to large' becomes 'l'.
'warps' back-to-front is 'spraw'.
'spraw'+'l'='SPRAWL'
(Other definitions for sprawl that I've seen before include "Loll, stretch out" , "Lounge" , "Sit in an ungainly manner" , "Sit lazily, with limbs spread wide" , "What stragglers do" .)