Support offered by soldiers aboard tender, so composed (11)
I believe the answer is:
endorsement
'tender so composed' is the definition.
I can't judge whether this defines the answer.
'support offered by soldiers aboard' is the wordplay.
'support' becomes 'endorse' ('endorse' can be a synonym of 'support').
'offered by' says to put letters next to each other.
'soldiers aboard' becomes 'ment' (I am not sure about this - if you are sure you should believe this answer much more).
'endorse'+'ment'='ENDORSEMENT'
Can you help me to learn more?
(Other definitions for endorsement that I've seen before include "Penalty appended to driving licence" , "drivers would prefer not to have?" , "unwelcome to offending motorists" , "Sanction, backing" , "Official sanction or approval" .)