Supported principal group of volunteers once in education (10)
I believe the answer is:
maintained
'supported' is the definition.
(I've seen this before)
'principal group of volunteers once in education' is the wordplay.
I cannot really understand how this works, but
'principal' could be 'main' (both can mean major) and 'main' is found within the answer.
'volunteers' could be 'ta' (abbreviation for Territorial Army) and 'ta' is found in the answer.
'in' is found in the answer.
'education' could be 'ed' (abbreviation - e.g. sex ed) and 'ed' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for maintained that I've seen before include "being kept going" , "Declared; supported" , "Insisted" , "Sustained" , "Kept up (level)" .)