Book showing self-restraint (7)
I believe the answer is:
reserve
I believe this clue is a double definition.
'book' is the first definition.
(as in booking or reserving in advance)
'self-restraint' is the second definition.
(thesaurus)
'showing' is the link.
(Other definitions for reserve that I've seen before include "Save" , "Order" , "Keep for future use" , "Reticence" , "Book - substitute player" .)