Hill-facing apartment failed to impress (4,4)
I believe the answer is:
fell flat
'failed to impress' is the definition.
(I've seen this before)
'hill-facing apartment' is the wordplay.
'hill' becomes 'fell' (I've seen this before).
'facing' means one lot of letters go next to another.
'apartment' becomes 'flat' (synonyms).
'fell'+'flat'='FELL FLAT'
(Other definitions for fell flat that I've seen before include "provoked no interest" , "collapsed" , "failed to impress" .)