Rodent from Welsh river runs, coming aboard coaster? (7)
I believe the answer is:
muskrat
'rodent' is the definition.
(I know that muskrat is a type of rodent)
'welsh river runs coming aboard coaster?' is the wordplay.
'welsh river' becomes 'usk' (River Usk in Wales).
'runs' becomes 'r' (cricket abbreviation).
'coming aboard' is an insertion indicator.
'coaster?' becomes 'mat' (I've seen this before).
'usk'+'r'='uskr'
'uskr' inserted within 'mat' is 'MUSKRAT'.
'from' is the link.
(Other definitions for muskrat that I've seen before include "Vole-like animal" , "Beaver-like rodent with glossy brown fur" , "Rodent prized for its fur" , "Large, smelly North American rodent" , "Beaver-like American rodent" .)