Propositions from soldiers bumping into quiet girls (9)
I believe the answer is:
premisses
'propositions' is the definition.
(premiss is a kind of proposition)
'soldiers bumping into quiet girls' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'bumping into' means one lot of letters goes inside another.
'quiet' becomes 'p' (music symbol for 'piano').
'girls' becomes 'misses' ('miss' can be a synonym of 'girl'**).
'p'+'misses'='pmisses'
're' placed within 'pmisses' is 'PREMISSES'.
'from' is the link.
(Another definition for premisses that I've seen is " underlying assumptions".)