Nervous individual periodically dogged (2,4)
I believe the answer is:
on edge
'nervous' is the definition.
(on edge can mean nervous or tense)
'individual periodically dogged' is the wordplay.
'individual' becomes 'one' (an individual is one person).
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The alternating letters of 'dogged' are 'dge'.
'one'+'dge'='ON EDGE'
(Other definitions for on edge that I've seen before include "Tense and irritable, with nerves stretched" , "Apprehensive" , "Anxious, nervy" , "Highly strung" , "Fidgety, nervous" .)