Docker perhaps not working is a sponger (10)
I believe the answer is:
freeloader
'a sponger' is the definition.
(synonyms)
'docker perhaps not working' is the wordplay.
I cannot really see how this works, but
'docker' could be 'loader' (synonyms) and 'loader' is present in the answer.
'not working' could be 'free' (I've seen this before) and 'free' is found in the answer.
No letters remain.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for freeloader that I've seen before include "Skinflint, say" , "One who gains from others' efforts" , "I'm a sucker" , "Sponger (informal)" , "One who takes but never gives" .)