Right limb, that thing (5)
I believe the answer is:
legit
'right' is the definition.
(both can mean legal)
'limb that thing' is the wordplay.
'limb' becomes 'leg' (leg is a kind of limb).
'that thing' becomes 'it'.
'leg'+'it'='LEGIT'
(Other definitions for legit that I've seen before include "true, in a word?" , "Authorised" , "ok!" , "Run away - above board (colloq)" , "Briefly in accordance with law or rules" .)