I dropped off tailored swimming costume (7)
I believe the answer is:
leotard
'costume' is the definition.
(costume worn by a dancer)
'i dropped off tailored swimming' is the wordplay.
'dropped off' is a deletion indicator.
'swimming' indicates anagramming the letters (letters swim around into a new order).
'tailored' with 'i' taken away is 'talored'.
'talored' anagrammed gives 'LEOTARD'.
(Other definitions for leotard that I've seen before include "Do alter the dancer's garment" , "Dance/exercise garment" , "Acrobat's costume" , "Garment used eg in ballet or yoga" , "Dance garment" .)