Justification of lay people on guitar periodically (8)
I believe the answer is:
apologia
'justification' is the definition.
(apologia is a kind of justification)
'lay people on guitar periodically' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'lay'+'people'+'on'+'guitar'='laypeopleonguitar'
The alternating letters of 'laypeopleonguitar' are 'APOLOGIA'.
'of' is the link.
(Other definitions for apologia that I've seen before include "form of defence" , "Formal defence of one's actions" , "Formal defence of belief or conduct" , "Formal defence of one's conduct" , "sort of defence" .)