Complain bitterly about the guard (4)
I believe the answer is:
rail
This clue could be a double definition.
'complain bitterly' is the first definition.
(I've seen this in another clue)
'the guard' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'about' acts as a link.
Can you help me to learn more?
(Other definitions for rail that I've seen before include "Barrier around lair" , "Line" , "Means of transport" , "Form of transport" , "Complain about - marsh bird" .)