Rotters needing a cool head when there's sad disarray (4)
I believe the answer is:
cads
'rotters' is the definition.
(I've seen this in another clue)
'a cool head when there's sad disarray' is the wordplay.
'a cool head when' becomes 'c' (I can't justify this - if you can you should believe this answer much more).
'there's' means one lot of letters go next to another.
'disarray' indicates an anagram.
'sad' anagrammed gives 'ads'.
'c'+'ads'='CADS'
'needing' is the link.
Can you help me to learn more?
(Other definitions for cads that I've seen before include "Rotters" , "Knaves" , "'Blackguards, bounders (4)'" .)