A constant trade union supporter in fact (8)
I believe the answer is:
actually
'in fact' is the definition.
(synonyms)
'a constant trade union supporter' is the wordplay.
'constant' becomes 'c' (common symbol in maths for an unknown constant).
'trade union' becomes 'TU' (abbreviation for trade union).
'supporter' becomes 'ally' (ally can mean a supporter or friend).
'a'+'c'+'tu'+'ally'='ACTUALLY'
(Other definitions for actually that I've seen before include "Really existing" , "In reality" , "A lay cult may be existing in fact" , "As a matter of fact" , "Really, in fact" .)