More than one serviceman remains unreliable (7)
I believe the answer is:
marines
'more than one serviceman' is the definition.
(marine is a kind of serviceman)
'remains unreliable' is the wordplay.
'unreliable' indicates an anagram.
'remains' anagrammed gives 'MARINES'.
(Other definitions for marines that I've seen before include "elite corps" , "Seminar for American troops" , "Naval service" , "Troops trained to serve on land or sea" , "forces" .)