Large heavier clothes more streamlined (7)
I believe the answer is:
flatter
'more streamlined' is the definition.
(flatter can mean more streamlined or smooth)
'large heavier clothes' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'heavier' becomes 'fatter' ('fat' can be similar in meaning to 'heavy').
'clothes' indicates putting letters inside (some letters clothe or cover others).
'l' put inside 'fatter' is 'FLATTER'.
(Other definitions for flatter that I've seen before include "Fawn, blandish" , "Compliment dishonestly" , "Praise insincerely" , "when all smoothed out?" , "more lifeless" .)