Diet of fruitcake limited term in service (9)
I believe the answer is:
nutriment
'diet' is the definition.
(I've seen this before)
'fruitcake limited term in service' is the wordplay.
'fruitcake' becomes 'nut' (I've seen this before).
'limited' says to take the centre (the limits of the word are removed).
'term' becomes 'period' (a term is a period of time).
'in service' becomes 'ment' (I can't justify this - if you can you should give a lot more credence to this answer).
The central letters of 'period' are 'ri'.
'nut'+'ri'+'ment'='NUTRIMENT'
'of' is the link.
Can you help me to learn more?
(Other definitions for nutriment that I've seen before include "Nourishing food" , "Sustenance, nourishment" , "It sustains" , "Material providing food" , "diet" .)