Smallest skirt mum put on (7)
I believe the answer is:
minimum
'smallest' is the definition.
(similar in meaning)
'skirt mum put on' is the wordplay.
'skirt' becomes 'mini' (a mini skirt).
'put on' means one lot of letters go next to another.
'mini'+'mum'='MINIMUM'
(Other definitions for minimum that I've seen before include "Lower limit" , "Nadir" , "Lowest point" , "Smallest possible quantity" , "Least recorded value" .)