Reason Scottish island line brought in charge? (9)
I believe the answer is:
rationale
'reason' is the definition.
(I've seen this before)
'scottish island line brought in charge?' is the wordplay.
'scottish island' becomes 'Iona' (island of the Hebrides).
'line' becomes 'l' (used when specifying particular lines from a poem).
'brought in' indicates putting letters inside.
'charge?' becomes 'rate' (rate is a kind of charge).
'iona'+'l'='ional'
'ional' placed within 'rate' is 'RATIONALE'.
(Other definitions for rationale that I've seen before include "solution in theory" , "Logical reasons (for action)" , "Logical reason for action" , "Reasoned exposition" , "This might account for" .)