Definition of one-trick pony noun from the Oxford Advanced Learner's Dictionary

 

one-trick pony

 noun
noun
BrE
 
; NAmE
 
(becoming old-fashioned, disapproving)
 
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a performer who is only famous for one song, etc.; a person or business that is only good at doing one thing This comedian is no one-trick pony. For the first six years of its life the company was a one-trick pony. But what a trick! We were a one-trick pony before, and now we’re able to provide a full range of services.
See the Oxford Advanced American Dictionary entry: one-trick pony