Most of repulsive regiment finally retreat (7)
I believe the answer is:
hideout
'retreat' is the definition.
(I've seen this before)
'most of repulsive regiment finally' is the wordplay.
'most of' means to remove the last letter.
'repulsive' becomes 'hideous' (I've seen this before).
'regiment finally' becomes 't' (I can't justify this - if you can you should give a lot more credence to this answer).
'hideous' with its last letter taken off is 'hideou'.
'hideou'+'t'='HIDEOUT'
(Other definitions for hideout that I've seen before include "sanctuary" , "retreat" , "Place to lie low" , "den" .)