Decline to have energy on books (3)
I believe the answer is:
ebb
'decline' is the definition.
(I know that ebb is a type of decline)
'energy on books' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mc2).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'books' becomes 'BB' (two abbreviations for book).
'e'+'bb'='EBB'
'to have' acts as a link.
(Other definitions for ebb that I've seen before include "(Water) recede" , "Withdraw heartless" , "Fall back" , "(Sea) recede" , "Go out like tide (3)" .)