Force cryptic clues on millions (6)
I believe the answer is:
muscle
'force' is the definition.
(I've seen this in another clue)
'cryptic clues on millions' is the wordplay.
'cryptic' indicates anagramming the letters (a cryptic or obscure order of the letters).
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'millions' becomes 'm' (abbreviation).
'clues' with letters rearranged gives 'uscle'.
'uscle' going around 'm' is 'MUSCLE'.
(Other definitions for muscle that I've seen before include "contractor" , "Eg, biceps" , "Physical power" , "meat" , "Brawn" .)