Rough diamond meeting Yankee (5)
I believe the answer is:
rocky
'rough' is the definition.
(similar in meaning)
'diamond meeting yankee' is the wordplay.
'diamond' becomes 'rock' (rock can informally mean a diamond).
'meeting' says to put letters next to each other.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'rock'+'y'='ROCKY'
(Other definitions for rocky that I've seen before include "lacking 15d [STABILITY]" , "Hard - unstable" , "Unsteady, unstable" , "Stony; unsteady" , "Unstable US mountain range?" .)