Right to trust missing book provides help (7)
I believe the answer is:
relieve
'help' is the definition.
(I've seen this before)
'right to trust missing book' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to trust' becomes 'believe' ('believe' can be a synonym of 'trust').
'missing' indicates named letters should be taken away.
'book' becomes 'b' (abbreviation for book).
'believe' with 'b' taken out is 'elieve'.
'r'+'elieve'='RELIEVE'
'provides' acts as a link.
(Other definitions for relieve that I've seen before include "Make less tedious, bring aid to (besieged place)" , "Break the sameness of" , "Assuage, ease" , "Make (pain) less severe" , "Soothe" .)