Sustenance provided by fresh trout in inn, tail removed (9)
I believe the answer is:
nutrition
'sustenance' is the definition.
(I've seen this before)
'fresh trout in inn tail removed' is the wordplay.
'fresh' is an anagram indicator (a fresh re-ordering of the letters).
'tail removed' means to remove the last letter.
'trout'+'in'+'inn'='troutininn'
'troutininn' with its last letter taken away is 'troutinin'.
'troutinin' with letters rearranged gives 'NUTRITION'.
'provided by' is the link.
(Other definitions for nutrition that I've seen before include "Food, nourishment" , "Food and drink, edibles" , "Act of nourishing" , "form of sustenance" , "what's taken in?" .)