Soldiers completely capturing small island, as is necessary (9)
I believe the answer is:
requisite
'is necessary' is the definition.
('requisite' can be similar in meaning to 'necessary')
'soldiers completely capturing small island' is the wordplay.
'soldiers' becomes 're' (Royal Engineers).
'completely' becomes 'quite' (I've seen this before**).
'capturing' indicates putting letters inside.
'small' becomes 's' (abbreviation - e.g. clothes size).
'island' becomes 'i'.
's'+'i'='si'
'quite' placed around 'si' is 'quisite'.
're'+'quisite'='REQUISITE'
'as' is the link.
(Other definitions for requisite that I've seen before include "Totally essential" , "Necessary" , "Indispensable" , "Needful" .)