Dirty old man's brother starts to annoy nurse (7)
I believe the answer is:
unclean
'dirty old man's' is the definition.
'unclean' can be an answer for 'dirty' (I've seen this before). I'm unsure of the 'old man's' bit.
'brother starts to annoy nurse' is the wordplay.
'brother' becomes 'uncle' (I have seen 'Brother of parent ' mean 'uncle' so perhaps 'brother' could also mean 'uncle').
'starts to' suggests taking the first letters.
The first letters of 'annoy nurse' is 'an'.
'uncle'+'an'='UNCLEAN'
Can you help me to learn more?
(Other definitions for unclean that I've seen before include "Dirty; immoral" , "(Ritually) tainted" , "Spotted" , "Dirty, needs a wash" , "Dirty, ritually impure" .)