Powerful fighter gains height over Yemeni 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 yemeni capital' is the wordplay.
'fighter' becomes 'mig' (fighter jet produced by Russian company MiG).
'gains' says to put letters next to each other.
'height' becomes 'ht' (abbreviation).
'over' says to put letters next to each other (in a down clue, some letters go over others).
'capital' suggests taking the first letters.
The initial letter of 'yemeni' is 'y'.
'mig'+'ht'+'y'='MIGHTY'
(Other definitions for mighty that I've seen before include "Arrogant people are often described as high and . . . . . ." , "Powerful - exalted" , "Having great strength" , "Extremely" , "Powerful - huge" .)