Subscribe a pound? That's notable (6)
I believe the answer is:
signal
'notable' is the definition.
(thesaurus)
'subscribe a pound?' is the wordplay.
'subscribe' becomes 'sign' ('sign' can be a synonym of 'subscribe').
'pound?' becomes 'l'.
'sign'+'a'+'l'='SIGNAL'
'that's' is the link.
(Other definitions for signal that I've seen before include "Outstanding - information sent over a distance" , "Noteworthy" , "Communication by sign" , "Important; communication" , "Important; cue" .)