Small and weak admitting run's gruelling (9)
I believe the answer is:
strenuous
'gruelling' is the definition.
('strenuous' can be a synonym of 'gruelling')
'small and weak admitting run's' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' says to put letters next to each other.
'weak' becomes 'tenuous' ('tenuous' can be similar in meaning to 'weak'**).
'admitting' indicates putting letters inside.
'run' becomes 'r' (cricket abbreviation).
'tenuous' going around 'r' is 'trenuous'.
's'+'trenuous'='STRENUOUS'
(Other definitions for strenuous that I've seen before include "Vigorous" , "Hard" , "Needing forceful exertion" , "Laborious" , "Physically demanding" .)