Inject a cubic centimetre into a climber as a safeguard (7)
I believe the answer is:
vaccine
'a safeguard' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'inject a cubic centimetre into a climber' is the wordplay.
'inject' becomes 'a' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a cubic centimetre' becomes 'cc' (synonyms).
'into' means one lot of letters goes inside another.
'a climber' becomes 'vine' (climbing plant).
'a'+'cc'='acc'
'acc' inserted into 'vine' is 'VACCINE'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for vaccine that I've seen before include "Antigenic preparation" , "anti-disease measure" , "Antibody stimulator" , "Preparation for inoculations" , "Immunity provider" .)