The object of a rough night? (5)
I believe the answer is:
thing
'the object' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'a rough night?' is the wordplay.
'a rough' indicates anagramming the letters (a rough or broken ordering of the letters).
'night' is an anagram of 'THING'.
'of' is the link.
Can you help me to learn more?
(Other definitions for thing that I've seen before include "See 1" , "Individual object" , "Night (anag)" , "Object; gadget" , "Obsession" .)