Definitely not forget after pilots picked up father (3,4,2)
I believe the answer is:
far from it
'definitely not' is the definition.
(phrase used to strongly deny a claim)
'forget after pilots picked up father' is the wordplay.
'forget' becomes 'omit' (to omit to do something is to forget to do it).
'after' says to put letters next to each other.
'pilots' becomes 'RAF' (Royal Air Force).
'picked up' says the letters should be written in reverse.
'father' becomes 'fr'.
'raf' written backwards gives 'far'.
'far'+'fr'='farfr'
'omit' put after 'farfr' is 'FAR FROM IT'.
(Other definitions for far from it that I've seen before include "Ma for rift? Most unlikely, too remote" , "Affirm rot? No way, quite remote" , "On the contrary" , "By no means" , "Well short of objective" .)