Aspiration about to be suppressed by mother (5)
I believe the answer is:
Here is my best explanation:
'aspiration' is the definition.
('dream' can be a synonym of 'aspiration')
'about to be suppressed by mother' is the subsidiary indication.
'about' becomes 're' (common abbreviation for 'with reference to').
'to be suppressed by' indicates putting letters inside (I've seen this in other clues).
'mother' becomes 'dam' (I've seen this before).
're' put within 'dam' is 'dream'.
(Other definitions for dream that I've seen before include "Ambition", "Nocturnal vision", "Fantasy; ideal", "Cherished hope; fantasy", "Sleeper's vision", "(Have) vision", "Nightmare", "Reverie or nightmare", "Somniate" and "Cherished desire".)