The first sailor allowed a notebook (6)
I believe the answer is:
tablet
'a notebook' is the definition.
(thesaurus)
'the first sailor allowed' is the wordplay.
'the first' means one lot of letters go next to another.
'sailor' becomes 'tab' (I am not sure about this - if you are sure you should believe this answer much more).
'allowed' becomes 'let' (to let is to allow or permit).
'tab' next to 'let' is 'TABLET'.
Can you help me to learn more?
(Other definitions for tablet that I've seen before include "Pill - plaque" , "Stone slab; pill" , "Small bar of soap" , "PC" , "Let bat provide a pill" .)