Rather fat runner, say, runs — just (8)
I believe the answer is:
matronly
'rather fat' is the definition.
(matronly can mean having a heavy build)
'runner say runs just' is the wordplay.
'runner say' becomes 'mat' (a runner is a type of rug or mat).
'runs' becomes 'r' (cricket abbreviation).
'just' becomes 'only' (synonyms).
'mat'+'r'+'only'='MATRONLY'
(Other definitions for matronly that I've seen before include "like some women!" , "Like a dignified or staid woman" , "like mum!" , "staid and plumpish" , "staid and podgy?" .)