Sign agreement — book available outside without question (2,5)
I believe the answer is:
no doubt
'without question' is the definition.
(I've seen this in another clue)
'sign agreement book available outside' is the wordplay.
'sign' becomes 'nod' (I have seen 'Sign of assent' mean 'nod' so perhaps 'sign' could also mean 'nod').
'agreement book' becomes 'b' (I am not sure about this - if you are sure you should believe this answer much more).
'available' becomes 'out' (I've seen this in other clues).
'outside' is an insertion indicator.
'b' inserted into 'out' is 'oubt'.
'nod'+'oubt'='NO DOUBT'
Can you help me to learn more?
(Other definitions for no doubt that I've seen before include "sign of agreement" , "To be sure" , "Surely" , "Almost certainly" , "Bound to, unquestionably" .)