Money maintains company known for flying fortress in Africa (6)
I believe the answer is:
casbah
'fortress in africa' is the definition.
(type of citadel or fortress in North Africa)
'money maintains company known for flying' is the wordplay.
'money' becomes 'cash' (cash is a type of money).
'maintains' is an insertion indicator.
'company known for flying' becomes 'BA' (British Airways).
'cash' going around 'ba' is 'CASBAH'.
(Other definitions for casbah that I've seen before include "Market place in N African towns" , "area round Algiers fortress" , "Market place in a N African town" , "Arab quarter" , "Native quarter of cities in northern Africa" .)