Final letters backed wise work plan (6)
I believe the answer is:
omegas
'final letters' is the definition.
(I've seen this in another clue)
'backed wise work plan' is the wordplay.
'backed' says the letters should be written in reverse.
'wise' becomes 'sage' (similar in meaning).
'work plan' becomes 'MO' (modus operandi).
'sage'+'mo'='sagemo'
'sagemo' in reverse letter order is 'OMEGAS'.
(Other definitions for omegas that I've seen before include "letters" , "Conclusions" , "Foreign characters" , "Greek characters'" .)