Senior citizen resold steroid in the centre (7)
I believe the answer is:
oldster
'senior citizen' is the definition.
(I've seen this before)
'resold steroid in the centre' is the wordplay.
'in the centre' says the answer is hidden in the clue.
'OLDSTER' is hidden within 'resold steroid'.
(Other definitions for oldster that I've seen before include "Elderly person" , "Greybeard" , "He's more experienced" , "A man getting old (colloq.)" , "One aged" .)