Attempt to support one American villain in theatre (4)
I believe the answer is:
iago
'villain in theatre' is the definition.
'iago' can be an answer for 'villain' (I've seen this before). I'm not sure about the 'in theatre' bit.
'attempt to support one american' is the wordplay.
'attempt' becomes 'go' (go is a kind of attempt).
'to support' says to put letters next to each other (in a down clue, some letters hold up others).
'one' becomes 'i' (Roman numeral).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'i'+'a'='ia'
'go' put after 'ia' is 'IAGO'.
Can you help me to learn more?
(Other definitions for iago that I've seen before include "Othello's enemy" , "Othello's evil counsellor" , "Evil character in Othello" , "Othello's Ancient" , "villain in play" .)