Periodically nomad seeks these sources of relief (5)
I believe the answer is:
oases
'these sources of relief' is the definition.
The definition suggests an adverb but the answer is not.
'periodically nomad seeks' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'nomad'+'seeks'='nomadseeks'
The alternating letters of 'nomadseeks' are 'OASES'.
Can you help me to learn more?
(Other definitions for oases that I've seen before include "Fertile areas in deserts" , "These sanctuaries" , "Green desert spots" , "Where there may be dates" , "Fertile desert spots" .)