Landed on island, in custody of Catholic queen's army (8)
I believe the answer is:
military
'army' is the definition.
(I know that army can be written as military)
'landed on island in custody of catholic queen's' is the wordplay.
'landed' becomes 'lit' (light or alight can mean to land).
'on' is a charade indicator (letters next to each other).
'island' becomes 'i' (geographical abbreviation).
'in custody of' indicates putting letters inside (inserted letters are taken into custody by others).
'catholic queen' becomes 'Mary' (Mary I).
'lit' after 'i' is 'ilit'.
'ilit' inserted within 'mary' is 'MILITARY'.
(Other definitions for military that I've seen before include "soldierly" , "in the field" , "Relating to the army" , "Relating to armed forces" , "Related to soldiering" .)