Sign that’s annoying about exercise wear (7)
I believe the answer is:
leotard
'exercise wear' is the definition.
'sign that's annoying about' is the wordplay.
'sign' becomes 'leo' (star sign).
'that's annoying' becomes 'drat' (expression of annoyance).
'about' says the letters should be written backwards.
'drat' in reverse letter order is 'tard'.
'leo'+'tard'='LEOTARD'
(Other definitions for leotard that I've seen before include "Acrobat costume" , "Dance garment" , "Dance/exercise garment" , "Ballet outfit" , "Acrobat's costume" .)