X interrupting frilly period of inactivity (7)
I believe the answer is:
latency
'period of inactivity' is the definition.
(I've seen this in another clue)
'x interrupting frilly' is the wordplay.
'x' becomes 'ten' (X is the Roman numeral for ten).
'interrupting' is an insertion indicator.
'frilly' becomes 'lacy' (I've seen this in other clues).
'ten' put within 'lacy' is 'LATENCY'.
(Other definitions for latency that I've seen before include "Potential" , "Hidden nature" , "Something that exists but is not yet developed" , "Time that elapses between a stimulus and the response to it" , "undiscovered state" .)