Sub has left for a bar (1,4)
I believe the answer is:
u bolt
'sub has left' is the definition.
The definition suggests a singular noun which matches the answer.
'a bar' is the wordplay.
I cannot really understand how this works, but
'bar' could be 'bolt' (bolt is a kind of bar) and 'bolt' is found in the answer.
A single letter 'u' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'for' acts as a link.
Can you help me to learn more?
(Other definitions for u bolt that I've seen before include "fastest-ever human" , "Fastener" , "one holds a connection" , "Means of fixing" , "Bent retainer" .)