Speak for Royal Engineers in attendance (9)
I believe the answer is:
represent
'speak for' is the definition.
(thesaurus)
'royal engineers in attendance' is the wordplay.
'royal engineers' becomes 'RE' (abbreviation).
'in attendance' becomes 'present' (similar in meaning).
're'+'present'='REPRESENT'
(Other definitions for represent that I've seen before include "Serve as a symbol for" , "Picture" , "Speak on behalf" , "Portray" , "show" .)