Setter had set on island, in a manner of speaking (5)
I believe the answer is:
idiom
'in a manner of speaking' is the definition.
(I've seen this before)
'setter had set on island' is the wordplay.
'setter had' becomes 'I'd' (I had - referring to the crossword setter).
'set on' says to put letters next to each other.
'island' becomes 'iom' (abbreviation for Isle of Man).
'id'+'iom'='IDIOM'
(Other definitions for idiom that I've seen before include "Saying specific to a certain language" , "characteristic use of language" , "Peculiar term" , "Language used by people or group, vernacular" , "Distinctive speech" .)