One involved in joint exercise (5)
I believe the answer is:
wield
'exercise' is the definition.
(thesaurus)
'one involved in joint' is the wordplay.
'one' becomes 'i' (Roman numeral).
'involved in' indicates putting letters inside.
'joint' becomes 'weld' (weld is a kind of joint).
'i' placed within 'weld' is 'WIELD'.
(Other definitions for wield that I've seen before include "exercise" , "Hold and use (a weapon or tool)" , "Brandish a weapon or exert power" , "Handle - manage" , "Use (weapon, authority)" .)