Short skirt with tight-lipped bottom (7)
I believe the answer is:
minimum
'bottom' is the definition.
(similar in meaning)
'short skirt with tight-lipped' is the wordplay.
'short skirt' becomes 'mini' (I've seen this before).
'with' says to put letters next to each other.
'tight-lipped' becomes 'mum' (I've seen this before).
'mini'+'mum'='MINIMUM'
(Other definitions for minimum that I've seen before include "Least possible quantity" , "Lower limit" , "Lowest value" , "Lowest point" , "Bottom" .)