Heavy books, most being possibly about the earth (5)
I believe the answer is:
tomes
'heavy books' is the definition.
(I've seen this in another clue)
'most being possibly about the earth' is the wordplay.
'being possibly' indicates an anagram.
'about' is an insertion indicator.
'the earth' becomes 'E'.
'most' anagrammed gives 'toms'.
'toms' enclosing 'e' is 'TOMES'.
(Other definitions for tomes that I've seen before include "Large scholarly books" , "Weighty books" , "Big books" , "works" .)