Active right gland (5)
I believe the answer is:
liver
'gland' is the definition.
(I've seen this before)
'active right' is the wordplay.
'active' becomes 'live' ('live' can be similar in meaning to 'active').
'right' becomes 'r' (common abbreviation).
'live'+'r'='LIVER'
(Other definitions for liver that I've seen before include "Large organ of body" , "Vital organ" , "Large organ in the abdomen secreting bile" , "Viler (anag.)" , "Bile-secreting organ" .)