Powerful fighter gains height over Yugoslav capital (6)
I believe the answer is:
mighty
'powerful' is the definition.
(I know that powerful can be written as mighty)
'fighter gains height over yugoslav capital' is the wordplay.
'fighter' becomes 'mig' (fighter jet produced by Russian company MiG).
'gains' means one lot of letters go next to another.
'height' becomes 'ht' (abbreviation).
'over' means one lot of letters go next to another.
'capital' suggests taking the first letters.
The initial letter of 'yugoslav' is 'y'.
'mig'+'ht'+'y'='MIGHTY'
(Other definitions for mighty that I've seen before include "Having great strength" , "Strong, powerful" , "Powerful - exalted" , "Very powerful/strong" , "Powerful - huge" .)