Dialect I'd picked up in the Isle of Man (5)
I believe the answer is:
idiom
'dialect' is the definition.
('idiom' can be a synonym of 'dialect')
'i'd picked up in the isle of man' is the wordplay.
'picked up' says the letters should be written in reverse.
'in the' means one lot of letters goes inside another.
'isle of man' becomes 'iom'.
'id' reversed gives 'di'.
'di' put inside 'iom' is 'IDIOM'.
(Other definitions for idiom that I've seen before include "Linguistic peculiarity" , "Characteristic mode of expression" , "Expression that does not make literal sense" , "Vocabulary characteristic of a specific group" , "Expression that's characteristic of particular group" .)