Caustic chap seen on trains getting agitated (10)
I believe the answer is:
astringent
'caustic' is the definition.
(thesaurus)
'chap seen on trains getting agitated' is the wordplay.
'chap' becomes 'gent' (I've seen this in other clues**).
'seen on' says to put letters next to each other.
'getting agitated' indicates anagramming the letters.
'trains' anagrammed gives 'astrin'.
'gent' after 'astrin' is 'ASTRINGENT'.
(Other definitions for astringent that I've seen before include "for checking bleeding" , "Harsh" , "biting" , "Styptic" , "Lotion" .)