The decline of electronic books (3)
I believe the answer is:
ebb
'the decline' is the definition.
(ebb is a kind of decline)
'electronic books' is the wordplay.
'electronic' becomes 'e' (as in e-mail or e-books).
'books' becomes 'bb'.
'e'+'bb'='EBB'
'of' is the link.
(Other definitions for ebb that I've seen before include "Withdraw heartless" , "recession" , "Abate" , "(Water) recede" , "Flow back or recede" .)