Worshipper of deity father left upset (6)
I believe the answer is:
lapdog
'worshipper' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'deity father left upset' is the wordplay.
'deity' becomes 'god' (a deity is a god or divine being).
'father' becomes 'pa' (pa can mean dad or father).
'left' becomes 'l' (common abbreviation).
'upset' says the letters should be written backwards.
'god'+'pa'+'l'='godpal'
'godpal' written backwards gives 'LAPDOG'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for lapdog that I've seen before include "One docile and subservient" , "One readily submitting" , "Submissive type" , "Much petted pet" , "Devoted attendant" .)