Next to assistant, hiding desires (9)
I believe the answer is:
alongside
'next to' is the definition.
(I've seen this before)
'assistant hiding desires' is the wordplay.
'assistant' becomes 'aide' (aide is a kind of assistant).
'hiding' indicates putting letters inside.
'desires' becomes 'longs' (longing is a kind of desiring).
'aide' placed around 'longs' is 'ALONGSIDE'.
(Other definitions for alongside that I've seen before include "Next to - alien dogs (anag)" , "Adjacent to" , "next door" , "Lying close to" , "Aligned so as to be next to, as ship moored to pier" .)