1. someone only skilled or successful in one area, or something only good for one particular purpose
he/she/it is a bit of a one-trick pony
Someone described as a one trick pony is a person who only has one particular skill or area of specialisation. Someone who is a one trick pony may not be very adaptable or be able to bring value to areas other than their realm of expertise. You might use this expression to say that even though someone has been successful or achieved a particular thing that you don't have confidence that they will be able to repeat their performance in another area.