Dialect I'd picked up in Isle of Man (5)
I believe the answer is:
idiom
'dialect' is the definition.
(synonyms)
'i'd picked up in isle of man' is the wordplay.
'picked up' says the letters should be written in reverse.
'in' is an insertion indicator.
'isle of man' becomes 'iom'.
'id' back-to-front is 'di'.
'di' put into 'iom' is 'IDIOM'.
(Other definitions for idiom that I've seen before include "Artistic style" , "Eg, over the moon" , "Peculiarity of phrase in a language" , "Distinctive expression" , "Characteristic style, vocabulary" .)