Severe strain, sadly, on fellow (10)
I believe the answer is:
astringent
'severe' is the definition.
(I've seen this before)
'strain sadly on fellow' is the wordplay.
'sadly' indicates anagramming the letters.
'on' means one lot of letters go next to another.
'fellow' becomes 'gent' (synonyms).
'strain' anagrammed gives 'astrin'.
'astrin'+'gent'='ASTRINGENT'
(Other definitions for astringent that I've seen before include "(Medication) causing contraction - stern giant (anag)" , "acerbic" , "medical preparation" , "(Of taste) sharp, bitter" , "Styptic" .)