Leading stronghold invaded by part of rising (8)
I believe the answer is:
foremost
'leading' is the definition.
(I've seen this before)
'stronghold invaded by part of rising' is the wordplay.
'stronghold' becomes 'fort' (I've seen this before).
'invaded by' is an insertion indicator.
'part of' becomes 'some' (only some of something).
'rising' is a reversal indicator (going up in a down clue).
'some' reversed gives 'emos'.
'fort' enclosing 'emos' is 'FOREMOST'.
(Other definitions for foremost that I've seen before include "Most prominent" , "First in rank" , "'Leading, best (8)'" , "top" , "First in time, place or importance" .)