Great secretary is a relative (7)
I believe the answer is:
grandpa
'relative' is the definition.
(I know that grandpa is a type of grandparent)
'great secretary' is the wordplay.
'great' becomes 'grand' (I've seen this before).
'secretary' becomes 'PA' (abbreviation for personal assistant).
'grand'+'pa'='GRANDPA'
'is a' acts as a link.
(Other definitions for grandpa that I've seen before include "Drag pan over to the elder" , "Veteran" , "family member" , "Parent's father" , "Pop" .)