Open a page mistakenly (5)
I believe the answer is:
agape
'open' is the definition.
(similar in meaning)
'a page mistakenly' is the wordplay.
'mistakenly' indicates an anagram.
'page' with letters rearranged gives 'gape'.
'a'+'gape'='AGAPE'
(Other definitions for agape that I've seen before include "Christian feast" , "Christian's love" , "the love of God!" , "Astonished" , "love a Christian shows" .)