Cow is close to dead and extremely thin - not good (5)
I believe the answer is:
daunt
'cow' is the definition.
(I've seen this in another clue)
'dead and extremely thin not good' is the wordplay.
'dead' becomes 'd' (abbreviation in government records).
'and' says to put letters next to each other.
'extremely thin' becomes 'gaunt' (I've seen this in another clue).
'not' is a deletion indicator.
'good' becomes 'g' (abbreviation).
'gaunt' with 'g' taken away is 'aunt'.
'd'+'aunt'='DAUNT'
'is close to' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for daunt that I've seen before include "Dishearten" , "Scare off - dismay" , "Cow" , "Make (someone) apprehensive" , "Discourage" .)