Cat eats old carp (4)
I believe the answer is:
moan
'carp' is the definition.
The definition and answer can be both to do with communicating as well as being verbs in their base form.
Maybe you can see an association between them that I can't see?
'cat eats old' is the wordplay.
'cat' becomes 'man' (I can't justify this - if you can you should believe this answer much more).
'eats' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'man' going around 'o' is 'MOAN'.
Can you help me to learn more?
(Other definitions for moan that I've seen before include "Express pain" , "Lament; grumble" , "Complain, groan" , "Keen" , "Murmur of pain; grumble" .)