Lifeline includes one display of enthusiasm (10)
I believe the answer is:
salivation
'enthusiasm' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'lifeline includes one' is the wordplay.
I cannot quite see how this works, but
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
The remaining letters 'salvation' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'display of' acts as a link.
Can you help me to learn more?
(Other definitions for salivation that I've seen before include "preparation for eating" , "sign of hunger" , "Signs of expecting lunch" , "Sign of anticipation" .)