Gang sometime ploughed up field of earth (12)

I believe the answer is:

geomagnetism

'field of earth' is the definition.
(the Earth's magnetic field)

'gang sometime ploughed up' is the wordplay.
'ploughed up' indicates anagramming the letters.
'gang'+'sometime'='gangsometime'
'gangsometime' with letters rearranged gives 'GEOMAGNETISM'.

(Another definition for geomagnetism that I've seen is " Branch of geology".)

I've seen this clue in The Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.