Fertiliser everyone can see fed to flipping old badger (5)
I believe the answer is:
guano
'fertiliser' is the definition.
(I've seen this before)
'everyone can see fed to flipping old badger' is the wordplay.
'everyone can see' becomes 'U' (as in a Universal film certificate).
'fed to' means one lot of letters goes inside another.
'flipping' says the letters should be written in reverse.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'badger' becomes 'nag' (both can mean to annoy or complain at).
'o'+'nag'='onag'
'onag' reversed gives 'gano'.
'u' placed into 'gano' is 'GUANO'.
(Other definitions for guano that I've seen before include "dried excrement" , "Bird droppings used as fertiliser" , "Touching month turning over" , "business to aid growth" , "Fertiliser (either from bird droppings or artificial)" .)