Flag enclosed, opened by relative (7)
I believe the answer is:
'flag' is the definition.
(I know that pennant is a type of flag)
'enclosed opened by relative' is the wordplay.
'enclosed' becomes 'pent' (I've seen this before).
'opened by' means one lot of letters goes inside another (one word creates an opening in another word).
'relative' becomes 'nan' (informal term for a grandmother).
'pent' going around 'nan' is 'PENNANT'.
(Other definitions for pennant that I've seen before include "Triangular flag" , "Standard" , "Tapering flag" , "Banner" , "Long narrow flag" .)