Invalid lain alert for three years (9)
I believe the answer is:
triennial
'years' is the definition.
The answer and definition can be both related to time as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'invalid lain alert for three' is the wordplay.
I cannot quite see how this works, but
an anagram of 'lain' is 'nial' which is within the answer.
'three' could be 'tri' (prefix meaning 'three') and 'tri' is found within the answer.
The remaining letters 'en' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for triennial that I've seen before include "Regularly occurring" , "Recurring every three years" , "lasting a few years" , "Occurring every three years" .)