In everyday usage, I would use the word amateur like this:
(Noun) I am an amateur rock climber. (Adjective) I participate in amateur rock climbing.
It also sounds right for me to say the following:
(1) (Adjective) I participate in, but am amateur at, rock climbing.
My question is whether the usage above is appropriate. In other words, using "amateur" in the form:
I am amateur at [...]
Oxford dictionary gives the example sentence "it's all so amateur!" that seems to have a similar usage.
It uses "amateur" as an adjective modifying "it", so it doesn't seem like too much of a stretch to have "amateur" modify "I". Is that enough evidence to support it being used in the above way?