Very strong players at the club
I believe the answer is:
cast iron
'very strong' is the definition.
(I've seen this before)
'players at the club' is the wordplay.
'players' becomes 'cast' (I've seen this before).
'at' means one lot of letters go next to another.
'the club' becomes 'iron' (iron is a kind of club**).
'cast'+'iron'='CAST-IRON'
(Other definitions for cast iron that I've seen before include "Brittle alloy" , "Extremely robust" , "not yielding" , "such an alibi?" , "Unyielding" .)