Pump up popular apartment price finally (7)
I believe the answer is:
inflate
'pump up' is the definition.
(I've seen this before)
'popular apartment price finally' is the wordplay.
'popular' becomes 'in'.
'apartment' becomes 'flat' ('flat' can be a synonym of 'apartment').
'finally' indicates one should take the final letters.
The last letter of 'price' is 'e'.
'in'+'flat'+'e'='INFLATE'
(Other definitions for inflate that I've seen before include "Enlarge" , "Blow up like balloon" , "Fill with gas" , "Cause to swell" , "Lean, fit (anag.)" .)