playing at (something)

How would you say using Latin American idiom "he only played at being an engineer"?

That phrase in Spanish would be:
(Él) sólo jugaba a ser ingeniero
o
(Él) sólo jugaba a que era ingeniero.

Él can be elided (and it is highly recomended to do it) if the person has previously been mentioned in the cotext.

I didn't get what you meant by "idiom". Is this an idiom in English??

Idiom means dialect. In this case, the way a Chilean might express the thought contrasted to the way a Mexican might express it.

Thanks greatly for your prompt reply.

I it refers to an adult person who acted as a person who knew a lot of enginery, "se las tiraba de ingeniero"
If It speaks about a child the former answers are correct

To express that idea in Argentina we say:
Se la da de ingeniero. (present, 3rd person singular)
Se la daba de ingeniero. (past tense, 3rd person singular)