It may be taken as an indication of health (11)
I believe the answer is:
temperature
'indication of health' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'it may be taken as an' is the wordplay.
I cannot quite see how this works, but
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
'an' could be 'per' (synonymous as in 'once an hour', 'once per hour') and 'per' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for temperature that I've seen before include "Degree of hotness" , "Measure of heat" , "Degree of heat" , "Measure of warmth" .)