Measurements of length, the very thing adopted by Baden-Powell’s groups (6)
I believe the answer is:
cubits
'measurements of length' is the definition.
(cubit is a historical unit of length)
'the very thing adopted by baden-powell's groups' is the wordplay.
'the very thing' becomes 'it'.
'adopted by' indicates putting letters inside (I've seen this in other clues).
'baden-powell's groups' becomes 'Cubs' (Robert Baden-Powell founded the Cubs and Scouts).
'it' going into 'cubs' is 'CUBITS'.
(Another definition for cubits that I've seen is " Ancient measures based on the length of the forearm".)