Sham flier heading north in gannet, acting like magpie? (9)

'acting like magpie?' is the definition.
Although both the answer and definition are present participle verbs, I don't understand how one could define the other.

'sham flier heading north in gannet' is the wordplay.
I cannot really understand how this works, but
an anagram of 'flier' is 'ilfer' which is located in the answer.
'north' could be 'n' (abbreviation) and 'n' is found in the answer.
The remaining letters 'pig' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?