After trains breaking down, man is caustic (10)
I believe the answer is:
astringent
'caustic' is the definition.
(thesaurus)
'after trains breaking down man' is the wordplay.
'after' is a charade indicator (letters next to each other).
'breaking down' is an anagram indicator.
'man' becomes 'gent' (gent is a kind of man**).
'trains' is an anagram of 'astrin'.
'astrin' next to 'gent' is 'ASTRINGENT'.
'is' acts as a link.
(Other definitions for astringent that I've seen before include "Lotion" , "Sharp or severe in manner" , "piercing" , "Harsh" , "description of wine?" .)