They joined the RAF in Berkshire (4)
I believe the answer is:
erks
'they joined the raf' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'in berkshire' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ERKS' can be found hidden inside 'berkshire'.
Can you help me to learn more?
(Other definitions for erks that I've seen before include "personnel in forces" , "Aircraftmen" , "fliers" , "chaps in forces" .)