Returning insect is eaten by another helplessly (6)

'helplessly' is the definition.

'returning insect is eaten by another' is the wordplay.
'returning' shows that the letters should be reversed in order.
'insect' becomes 'bee' (bee is a kind of insect).
'is eaten by' means one lot of letters goes inside another.
'another' becomes 'fly' (I can't explain this - if you can you should believe this answer much more).
'bee' in reverse letter order is 'eeb'.
'eeb' put within 'fly' is 'FEEBLY'.