Fit one new compartment (7)
I believe the answer is:
install
'fit' is the definition.
(thesaurus)
'one new compartment' is the wordplay.
'one new' becomes 'in' (I've seen this in other clues).
'compartment' becomes 'stall' (I have seen 'Stable compartment ' mean 'stall' so perhaps 'compartment' could also mean 'stall').
'in'+'stall'='INSTALL'
(Other definitions for install that I've seen before include "place in office" , "Settle" , "Set up, ready for use" , "Put in place ready for use" , "Place in position or rank" .)