Fellow always has ague (5)
I believe the answer is:
fever
'ague' is the definition.
(I've seen this before)
'fellow always' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'always' becomes 'ever' (synonyms).
'f'+'ever'='FEVER'
'has' is the link.
(Other definitions for fever that I've seen before include "State of heightened excitement" , "Masefield's desire for the sea" , "A rise in the temperature of the body" , "A rise in the temperature from infection" , "Nervous excitement, high temperature" .)