Check around the front of son's gum (5)
I believe the answer is:
resin
'gum' is the definition.
(thesaurus)
'check around the front of son's' is the wordplay.
'check' becomes 'rein' (both can mean to control or limit).
'around' means one lot of letters goes inside another.
'the front of' suggests taking the first letters.
The initial letter of 'sons' is 's'.
'rein' going around 's' is 'RESIN'.
(Other definitions for resin that I've seen before include "Sticky stuff oozing from trees" , "Sticky stuff for siren" , "Viscous substance in some trees" , "It exudes from pines" , "Material exuded from tree" .)