Son that is returning gym skirt (8)
I believe the answer is:
sidestep
'skirt' is the definition.
('sidestep' can be a synonym of 'skirt')
'son that is returning gym' is the wordplay.
'son' becomes 's'.
'that is' becomes 'idest' ('id est' can be a synonym of 'that is').
'returning' says the letters should be written in reverse.
'gym' becomes 'pe'.
'pe' back-to-front is 'ep'.
's'+'idest'+'ep'='SIDESTEP'
(Other definitions for sidestep that I've seen before include "Dodge - circumvent" , "Dodge (an issue?)" , "Find a way round" , "Shift to right or left" , "To avoid" .)