The writing of letters, for instance (5)
I believe the answer is:
essay
'the writing' is the definition.
(essay is a kind of writing)
'letters for instance' is the wordplay.
'letters' becomes 'es' (more than one E).
'for instance' becomes 'say' (both can be used to introduce an example).
'es'+'say'='ESSAY'
'of' is the link.
(Other definitions for essay that I've seen before include "Make an effort or attempt" , "Try; dissertation" , "Short formal piece of writing" , "Written academic work" , "Prose composition" .)