Scout, a posh one, ridiculously watchful (8)
I believe the answer is:
cautious
'watchful' is the definition.
(similar in meaning)
'scout a posh one ridiculously' is the wordplay.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'one' becomes 'i' (Roman numeral).
'ridiculously' indicates anagramming the letters.
'scout'+'a'+'u'+'i'='scoutaui'
'scoutaui' with letters rearranged gives 'CAUTIOUS'.
(Other definitions for cautious that I've seen before include "Careful, shrewd" , "Chary of" , "Wary or prudent" , "Prudent, slow to take a chance" , "Careful to avoid dangers" .)