Encroachment sailor oddly admitted to leading? (8)
I believe the answer is:
'encroachment' is the definition.
'sailor oddly admitted to leading?' is the wordplay.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'admitted to' indicates putting letters inside.
'leading?' becomes 'invan' (van can mean the leading part of a group).
The alternating letters of 'sailor' are 'sio'.
'sio' put inside 'invan' is 'INVASION'.
(Other definitions for invasion that I've seen before include "Occupation" , "Incursive attack" , "Offensive" , "Unwelcome encroachment on another country" , "Encroachment by an army" .)