Modify commercial apartment (5)
I believe the answer is:
adapt
'modify' is the definition.
(thesaurus)
'commercial apartment' is the wordplay.
'commercial' becomes 'ad'.
'apartment' becomes 'apt' (abbreviation in property ads for example).
'ad'+'apt'='ADAPT'
(Other definitions for adapt that I've seen before include "Adjust to new circumstances" , "Become adjusted to new conditions" , "Adjust to changed circumstances" , "match" , "Convert to another use or situation" .)