Outfit conquers sweetheart in double time (7)
I believe the answer is:
twinset
'outfit' is the definition.
(twinset is a type of outfit)
'conquers sweetheart in double time' is the wordplay.
'conquers' becomes 'wins' (I've seen this in another clue).
'sweetheart' becomes 'e' (the middle letter or heart of 'sweet').
'in' means one lot of letters goes inside another.
'double' says to double the letters.
'time' becomes 't' (abbreviation).
'wins'+'e'='winse'
't' duplicated is 'tt'.
'winse' inserted inside 'tt' is 'TWINSET'.
(Other definitions for twinset that I've seen before include "Matching garments" , "Wear associated with pearls?" , "Matching cardigan and jumper when worn at the same time" , "Kit worn to match" , "clothing" .)