Gyrating part of body in fat dancer's costume (7)
I believe the answer is:
leotard
'dancer's costume' is the definition.
'gyrating part of body in fat' is the wordplay.
'gyrating' says the letters should be written in reverse.
'part of body' becomes 'toe'.
'in' is an insertion indicator.
'fat' becomes 'lard' (I've seen this before).
'toe' written backwards gives 'eot'.
'eot' placed within 'lard' is 'LEOTARD'.
(Other definitions for leotard that I've seen before include "Acrobat's costume" , "Close-fitting garment worn by dancers" , "Stretchy one-piece garment" , "Garment worn by ballet dancers" , "Dance garment" .)