Lucky Sam, going back to bed (6)
I believe the answer is:
mascot
'lucky' is the definition.
The answer and definition are not the same part of speech. However, I know that 'lucky' relates to this answer.
'sam going back to bed' is the wordplay.
'going back' says the letters should be written in reverse.
'to' says to put letters next to each other.
'bed' becomes 'cot' (cot is a kind of bed).
'sam' back-to-front is 'mas'.
'mas'+'cot'='MASCOT'
Can you help me to learn more?
(Other definitions for mascot that I've seen before include "good luck charm required" , "Lucky person" , "Something considered to bring good luck" , "Symbolic figure, of a team perhaps" , "Thing believed to bring good luck to, say, team" .)