Splendid secretary who’s been in the family a long time? (7)
I believe the answer is:
Here is my best explanation:
'who's been in the family a long time?' is the definition.
'splendid secretary' is the subsidiary indication.
'splendid' becomes 'grand' (I've seen this in other clues).
'secretary' becomes 'pa' (I've seen this in other clues).
(Other definitions for grandpa that I've seen before include "much older relative?", "Old man", "Parent's father", "Drag pan over to the elder", "Older person", "family member", "Relation", "Veteran", "A parent's father" and "Pop".)