Do alter unusually tight-fitting garment (7)
I believe the answer is:
leotard
'tight-fitting garment' is the definition.
(I've seen this before)
'do alter unusually' is the wordplay.
'unusually' is an anagram indicator.
'do'+'alter'='doalter'
'doalter' is an anagram of 'LEOTARD'.
(Other definitions for leotard that I've seen before include "Dance garment" , "Garment for gymnast" , "Gymnast's tight costume" , "Acrobat's costume" , "Stretchy one-piece garment" .)