Going up in plane, see giant web-footed birds (5)
I believe the answer is:
geese
'web-footed birds' is the definition.
(I've seen this in another clue)
'going up in plane see giant' is the wordplay.
'going up' says the letters should be written backwards (in a down clue, the letters are written upwards).
'in' says the answer is hidden in the clue.
'eseeg' can be found hidden inside 'plane see giant'.
'eseeg' reversed gives 'GEESE'.
(Other definitions for geese that I've seen before include "Stupid people" , "Long-necked, migratory aquatic birds" , "Birds in a gaggle" , "Large birds of the duck family" , "Protectors of city" .)