Unconnected Irish soldiers allowed to secure transport (10)
I believe the answer is:
irrelevant
'unconnected' is the definition.
(thesaurus)
'irish soldiers allowed to secure transport' is the wordplay.
'irish' becomes 'IR' (abbreviation).
'soldiers' becomes 'RE' (Royal Engineers).
'allowed to' becomes 'let' (to let something happen is to allow it).
'secure' indicates putting letters inside (inserted letters are secured inside).
'transport' becomes 'van' (van is a kind of transport).
'let' enclosing 'van' is 'levant'.
'ir'+'re'+'levant'='IRRELEVANT'
(Other definitions for irrelevant that I've seen before include "Not material" , "Not pertinent" , "Impertinent" , "Beside the point" , "Immaterial" .)