Yankee interferes without mass agreements (6)
I believe the answer is:
yesses
'agreements' is the definition.
(more than one yes)
'yankee interferes without mass' is the wordplay.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'interferes' becomes 'messes' (to interfere with is to mess with).
'without' is a deletion indicator.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'messes' with 'm' removed is 'esses'.
'y'+'esses'='YESSES'
(Other definitions for yesses that I've seen before include "Votes in favour" , "Affirmatives" .)