This evening, son's working out in pantihose (6)
I believe the answer is:
tights
'pantihose' is the definition.
(I've seen this before)
'this evening son's working out' is the wordplay.
'this evening' becomes 'tonight' ('tonight' can be a synonym of 'this evening').
'working' becomes 'on'.
'out' suggests deleting specific letters.
'tonight'+'s'='tonights'
'tonights' with 'on' taken out is 'TIGHTS'.
'in' acts as a link.
(Other definitions for tights that I've seen before include "Women's garments" , "Ballet wear" , "Close fitting garment" , "form of legwear" , "Pantihose" .)