Mild-mannered chap the Spanish upset (6)
I believe the answer is:
gentle
'mild-mannered' is the definition.
(I've seen this before)
'chap the spanish upset' is the wordplay.
'chap' becomes 'gent' (both can mean a man).
'the spanish' becomes 'el' ('the' in Spanish).
'upset' shows that the letters should be reversed in order.
'el' reversed gives 'le'.
'gent'+'le'='GENTLE'
(Other definitions for gentle that I've seen before include "that may help angler catch fish" , "Kind-natured" , "Soft and mild, quiet and soothing" , "Kind, not harsh" , "Not violent, tender" .)