Flies large balloon into refuelling areas (6)
I believe the answer is:
pilots
'flies' is the definition.
(I know this)
'large balloon into refuelling areas' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'balloon' becomes 'O' (O resembles a balloon).
'into' is an insertion indicator.
'refuelling areas' becomes 'pits' (I am not sure about this - if you are sure you should believe this answer much more).
'l'+'o'='lo'
'lo' inserted into 'pits' is 'PILOTS'.
Can you help me to learn more?
(Other definitions for pilots that I've seen before include "Fliers or guides" , "Shepherds" , "They guide ships in and out of harbours" , "Aircraft operators" , "Aviators and ships' guides" .)