A number eleven entering field of play, hungry-looking? (8)
I believe the answer is:
anorexic
'hungry-looking?' is the definition.
'a number eleven entering field of play' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'eleven' becomes 'xi' (roman numeral).
'entering' means one lot of letters goes inside another.
'field of play' becomes 'rec' (short for recreation ground).
'xi' placed into 'rec' is 'rexic'.
'a'+'no'+'rexic'='ANOREXIC'
(Other definitions for anorexic that I've seen before include "Person with obsessive food disorder" , "Suffering from loss of appetite" , "Suffering from a prolonged eating disorder" .)