Likely to keep book on rugby short (6)
I believe the answer is:
abrupt
'short' is the definition.
(thesaurus)
'likely to keep book on rugby' is the wordplay.
'likely' becomes 'apt' ('apt' can be similar in meaning to 'likely').
'to keep' means one lot of letters goes inside another.
'book' becomes 'b'.
'on' says to put letters next to each other.
'rugby' becomes 'ru'.
'b'+'ru'='bru'
'apt' going around 'bru' is 'ABRUPT'.
(Other definitions for abrupt that I've seen before include "Curt and brief" , "Sudden; hasty, curt" , "Sudden, maybe discourteous" , "Impolite" , "Curt; precipitous" .)