It's yellow getting Trotsky around Mexican capital (5)
I believe the answer is:
lemon
'it's yellow' is the definition.
(lemon is a kind of yellow)
'trotsky around mexican capital' is the wordplay.
'trotsky' becomes 'leon' (I've seen this before).
'around' indicates putting letters inside.
'capital' says to take the initial letters.
The initial letter of 'mexican' is 'm'.
'leon' enclosing 'm' is 'LEMON'.
'getting' acts as a link.
(Other definitions for lemon that I've seen before include "Fruit - as an answer it's unsatisfactory" , "something unattractive" , "Fruit-tree" , "Pale yellow, as of citrus fruit" , "Strong yellow colour" .)