Short book? Star restricts length (7)
I believe the answer is:
novella
'short book?' is the definition.
(a short novel)
'star restricts length' is the wordplay.
'star' becomes 'nova' (nova is a kind of star).
'restricts' indicates putting letters inside.
'length' becomes 'ell' (I've seen this before).
'nova' enclosing 'ell' is 'NOVELLA'.
(Other definitions for novella that I've seen before include "story, short one" , "short book" , "piece of fiction" , "Short fictional work" , "Short fictional piece" .)