Regularly engage poacher needing this? (3)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

Alternate letters of E n G a G e – or so I thought, until I looked at the comments on the Guardian thread, advising the use of the ‘reveal’ button and found that the ‘correct’ answer was ERG; I can’t find any explanation of either the definition or the wordplay (Stop Press: it has now been changed to EGG)