A little chap (but not if it's Schwarzenegger) (5)

I believe the answer is:

arnie

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'it's schwarzenegger' is the definition.
(Arnold Schwarzenegger is nicknamed Arnie)

I cannot understand how the rest of the clue works.

(Other definitions for arnie that I've seen before include "Arnold (abbrev.)" , "Mr Schwarzenegger, familiarly" .)

I've seen this clue in the Evening Standard.

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