Silverback training to open mouth wide (4)
I believe the answer is:
gape
'mouth wide' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'silverback training to open' is the wordplay.
I cannot really see how this works, but
'training' could be 'pe' (short for Physical Education) and 'pe' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for gape that I've seen before include "Failure to meet" , "Look amazed" , "Peer" , "Look in amazement or stupidity" , "Gawp" .)