Cost of life expectancy (3)
I believe the answer is:
fee
'cost' is the definition.
(fee is a kind of cost)
'of life expectancy' is the wordplay.
'of' indicates a hidden word.
'FEE' is hidden in the letters of 'life expectancy'.
(Other definitions for fee that I've seen before include "Money paid for professional services" , "Sum payable to, say, barrister" , "Sum payable to professional person for services" , "price" , "Payment to a professional for services" .)