Boat's a runner (very loud) (5)
I believe the answer is:
'boat's' is the definition.
(skiff is a kind of boat)
'a runner very loud' is the wordplay.
'a runner' becomes 'ski' (ski is a kind of runner).
'very loud' becomes 'ff' (musical abbreviation).
(Other definitions for skiff that I've seen before include "Small light boat" , "Light rowing boat for one person" , "Small rowing boat" .)