Suitable to include Royal Society in describing this clue (5)
I believe the answer is:
first
'this clue' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'suitable to include royal society in' is the wordplay.
'suitable' becomes 'fit' (similar in meaning).
'to include' indicates putting letters inside.
'royal' becomes 'r' (abbreviation).
'society' becomes 's' (common abbreviation in names of organisations).
'in' means one lot of letters goes inside another.
'fit' enclosing 'r' is 'firt'.
'firt' going around 's' is 'FIRST'.
'describing' acts as a link.
(Other definitions for first that I've seen before include "in dominant position" , "Initial" , "Preliminary" , "higher than the others" , "Coming before all others" .)