Girlfriend eating eggs was a mistake (4)
I believe the answer is:
goof
'a mistake' is the definition.
(I've seen this in another clue)
'girlfriend eating eggs' is the wordplay.
'girlfriend' becomes 'gf' (common abbreviation for girlfriend).
'eating' indicates putting letters inside.
'eggs' becomes 'oo' (two O's - O looks like an egg).
'gf' going around 'oo' is 'GOOF'.
'was' is the link.
(Other definitions for goof that I've seen before include "boob" , "Blunder, mistake (slang)" , "(Make a) silly mistake" , "Foolish mistake" , "In slang, bungle or blunder" .)