Young dog with one left eye? (5)
I believe the answer is:
pupil
'eye?' is the definition.
(thesaurus)
'young dog with one left' is the wordplay.
'young dog' becomes 'pup' (short for puppy).
'with' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'left' becomes 'l' (common abbreviation).
'pup'+'i'+'l'='PUPIL'
(Other definitions for pupil that I've seen before include "Centre of iris, in the eye" , "One being taught" , "School learner" , "Eye's centre" , "I learn" .)