Punctuality needed to supply lines to one before strange duet (11)
I believe the answer is:
promptitude
'punctuality' is the definition.
(thesaurus)
'supply lines to one before strange duet' is the wordplay.
'supply lines' becomes 'prompt' (I've seen this before).
'to one' becomes 'i' (Roman numeral).
'before' says to put letters next to each other.
'strange' indicates an anagram.
'duet' is an anagram of 'tude'.
'prompt'+'i'+'tude'='PROMPTITUDE'
'needed to' acts as a link.
(Other definitions for promptitude that I've seen before include "Detroit pump (anag) - readiness to act" , "Quickness of action; readiness" .)