Variable in flight (6)
I believe the answer is:
yawing
'variable in flight' is the definition.
Although both the answer and 'flight' are verbs, they are not in the same form of the verb.
'variable in flight' is the wordplay.
'variable' becomes 'y' (algebra).
'in flight' becomes 'a-wing' (I've seen this in another clue).
'y'+'awing'='YAWING'
Can you help me to learn more?
(Other definitions for yawing that I've seen before include "sailing unsteadily" , "flying erratically" .)