Sailor put up valid proposition (7)
I believe the answer is:
solicit
'proposition' is the definition.
(I've seen this before)
'sailor put up valid' is the wordplay.
'sailor' becomes 'os' (ordinary seaman).
'put up' says the letters should be written backwards.
'valid' becomes 'licit'.
'os' reversed gives 'so'.
'so'+'licit'='SOLICIT'
(Other definitions for solicit that I've seen before include "Ask (someone) for something" , "Drum up business" , "Ask earnestly about colitis" , "Request urgently and/or persistently" , "Request, canvas" .)