Invariably costly meal, it's said, extended by indefinite period (8)
I believe the answer is:
luncheon
'invariably costly meal' is the definition.
'luncheon' can be an answer for 'meal' (luncheon is a kind of meal). I'm not certain of the 'invariably costly' bit.
'said extended by indefinite period' is the wordplay.
I cannot quite understand how this works, but
'period' could be 'eon' (eon is a kind of period) and 'eon' is present in the answer.
The remaining letters 'lunch' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'it's' is the link.
Can you help me to learn more?
(Other definitions for luncheon that I've seen before include "Midday meal" , "Food" , "formal occasion" , "fare at one?" , "midday menu" .)