Foolish time to put identifying mark into deer (6)
I believe the answer is:
dotage
'foolish time' is the definition.
The definition and answer can be both related to time as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'identifying mark into deer' is the wordplay.
'identifying mark' becomes 'tag' (tag is a kind of mark. I am not sure about the 'identifying' bit.).
'into' is an insertion indicator.
'deer' becomes 'doe' (female deer).
'tag' placed within 'doe' is 'DOTAGE'.
'to put' is the link.
Can you help me to learn more?
(Other definitions for dotage that I've seen before include "Old age, senility" , "Second childhood" , "State of weakness of mind caused by old age" , "Mental infirmity in old age" , "Old and weak time of life" .)