Informer, so they say, getting short prison sentence (1,6,4)
I believe the answer is:
a little bird
'informer so they say' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'short prison sentence' is the wordplay.
I cannot really understand how this works, but
'short' could be 'little' (similar in meaning) and 'little' is found in the answer.
'prison' could be 'bird' and 'bird' is located in the answer.
A single letter 'a' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'getting' is the link.
(Other definitions for a little bird that I've seen before include "one's secret informer" , "Robin could be described as" .)