Single knocker, we're told (11)
I believe the answer is:
opportunity
'knocker we're told' is the definition.
I can't judge whether this definition defines the answer.
'single' is the wordplay.
I cannot really understand how this works, but
'single' could be 'unity' (synonyms) and 'unity' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for opportunity that I've seen before include "Prospect" , "One may knock" , "25 28 [MARS LANDER]" , "knocker?" , "It only knocks once" .)