Ardent supporter outside in field (10)
I believe the answer is:
discipline
'field' is the definition.
(I know that field can be written as discipline)
'ardent supporter outside in' is the wordplay.
I cannot really understand how this works, but
'in' is within the answer.
The remaining letters 'disciple' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for discipline that I've seen before include "Correct" , "Field" , "Correction" , "Branch of knowledge; punish" , "Training designed to engender self-control" .)