Change mind about Sappers being disloyal (8)
I believe the answer is:
recreant
'disloyal' is the definition.
(similar in meaning)
'change mind about sappers' is the wordplay.
'change mind' becomes 'recant' (as in recanting a confession).
'about' means one lot of letters goes inside another.
'sappers' becomes 'RE' (nickname for the Royal Engineers).
'recant' enclosing 're' is 'RECREANT'.
'being' acts as a link.
(Other definitions for recreant that I've seen before include "disloyal" , "Craven, apostate" , "Cowardly; apostate" , "yellow" .)