Regions taking off on a cause (5)
I believe the answer is:
areas
'regions' is the definition.
(I know this)
'taking off on a cause' is the wordplay.
'taking off' suggests deleting specific letters.
'cause' becomes 'reason' (I've seen this before).
'a'+'reason'='areason'
'areason' with 'on' taken away is 'AREAS'.
(Other definitions for areas that I've seen before include "Places" , "Zones" , "Localities" , "Geographical regions" , "Spaces covered by two-dimensional surfaces" .)