Substantial addition to pure alcohol (4)
I believe the answer is:
real
'substantial' is the definition.
(synonyms)
'addition to pure alcohol' is the wordplay.
'addition to' indicates a hidden word (the answer is an addition to the clue).
'REAL' is hidden within 'pure alcohol'.
(Other definitions for real that I've seen before include "'Genuine, actual (4)'" , "Actual and tangible" , "Genuine, not imitation" , "Authentic" , "Unaffected" .)