One on coach by Scottish loch causing concern (8)
I believe the answer is:
business
'concern' is the definition.
(business is a kind of concern)
'one on coach by scottish loch' is the wordplay.
'one' becomes 'i' (Roman numeral).
'on' means one lot of letters go next to another.
'coach' becomes 'bus' ('bus' can be a synonym of 'coach'**).
'by' means one lot of letters go next to another.
'scottish loch' becomes 'ness' (I've seen this before).
'i' after 'bus' is 'busi'.
'busi'+'ness'='BUSINESS'
'causing' acts as a link.
(Other definitions for business that I've seen before include "Work - shop" , "Affair - matters to be attended to" , "See 1" , "Commerce, trade" , "complicated problem" .)