Acquiring new apartment? That's swell! (7)
I believe the answer is:
inflate
'swell' is the definition.
(I've seen this before)
'acquiring new apartment? that's' is the wordplay.
'acquiring' means one lot of letters goes inside another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'apartment?' becomes 'flat' (synonyms).
'that's' becomes 'ie'.
'n'+'flat'='nflat'
'nflat' put into 'ie' is 'INFLATE'.
(Other definitions for inflate that I've seen before include "Exaggerate or fill with air" , "Fill with gas" , "aggrandize them" , "Cause to swell" , "Blow up like balloon" .)