Power of aircraft gaining height with time (5)
I believe the answer is:
might
'power' is the definition.
(I know that power can be written as might)
'aircraft gaining height with time' is the wordplay.
'aircraft' becomes 'mig' (I have seen 'Russian aircraft ' mean 'mig' so perhaps 'aircraft' could also mean 'mig').
'gaining' is a charade indicator (letters next to each other).
'height' becomes 'h' (abbreviation in geometry).
'with' is a charade indicator (letters next to each other).
'time' becomes 't' (abbreviation).
'mig'+'h'+'t'='MIGHT'
'of' acts as a link.
(Other definitions for might that I've seen before include "What functions as an auxiliary" , "Clout" , "May, in the past" , "word expressing possibility" , "It expresses possibility" .)