Private’s unusual solar pen (8)
I believe the answer is:
personal
'private's' is the definition.
(associated in meaning)
'unusual solar pen' is the wordplay.
'unusual' indicates an anagram.
'solar'+'pen'='solarpen'
'solarpen' with letters rearranged gives 'PERSONAL'.
(Other definitions for personal that I've seen before include "Such a rude remark" , "Particular to a given individual" , "Concerning oneself" , "Distinctive; individual" , "Private - (of remarks) offensive" .)