Fix son's legs (4)
I believe the answer is:
pins
'legs' is the definition.
(pin is a kind of leg)
'fix son's' is the wordplay.
'fix' becomes 'pin' (pinning is a kind of fixing**).
'pin'+'s'='PINS'
(Other definitions for pins that I've seen before include "With needles, they create a tingling sensation" , "Wire fasteners" , "security codes" , "Stocking fillers" .)