Dull English or US coin (4)
I believe the answer is:
dime
'us coin' is the definition.
(US coin worth ten cents)
'dull english' is the wordplay.
'dull' becomes 'dim' (synonyms).
'english' becomes 'e' (abbreviation).
'dim'+'e'='DIME'
'or' is the link.
(Other definitions for dime that I've seen before include "Ten cents in USA" , "US 10 cent coin" , "US ten cent coin" , "10c coin" , "Ten cents piece" .)