It's usual to find a bit in a mixed haul (8)
I believe the answer is:
habitual
'it's usual' is the definition.
(I've seen this before)
'a bit in a mixed haul' is the wordplay.
'in' indicates putting letters inside.
'a mixed' is an anagram indicator.
'a'+'bit'='abit'
'haul' with letters rearranged gives 'hual'.
'abit' inserted inside 'hual' is 'HABITUAL'.
'to find' acts as a link.
(Other definitions for habitual that I've seen before include "A halibut? (anag)" , "Done constantly by a halibut" , "Commonly practised" , "Like a routine" , "Chronic" .)