Punch's dog needs to get by (4)
I believe the answer is:
toby
'punch's dog' is the definition.
'toby' can be an answer for 'dog' (I've seen this before). I am not sure about the 'punch's' bit.
'to get by' is the wordplay.
I cannot really see how this works, but
'to' is found in the answer.
'by' is present in the answer.
No letters remain.
This explanation may well be incorrect...
'needs' acts as a link.
Can you help me to learn more?
(Other definitions for toby that I've seen before include "Member of the 12 [SHANDY] family" , "fellow" , "So-called knight" , "dog with frill round neck" , "Kind of jug" .)