Thread holding large key (5)
I believe the answer is:
lisle
'thread' is the definition.
(lisle is a kind of thread)
'large key' is the wordplay.
'large' becomes 'l'.
'key' becomes 'isle' (I've seen this before).
'l'+'isle'='LISLE'
'holding' is the link.
(Other definitions for lisle that I've seen before include "Fine cotton thread used in stockings" , "Pedestrianised city" , "Fine smooth cotton thread for stockings" , "Type of cotton yarn" , "(Fabric woven with) a fine cotton thread" .)