Support below leg (8)
I believe the answer is:
underpin
'support' is the definition.
(I know this)
'below leg' is the wordplay.
'below' becomes 'under' (synonyms).
'leg' becomes 'pin' (pin is a kind of leg).
'under'+'pin'='UNDERPIN'
(Other definitions for underpin that I've seen before include "corroborate" , "Form the basis for" , "buttress" , "Support or strengthen from beneath" , "Prop up with nun pride" .)