Soldiers' mess showing The Magnificent Seven perhaps (6)
I believe the answer is:
rehash
'the magnificent seven perhaps' is the definition.
I can't judge whether this defines the answer.
'soldiers' mess' is the wordplay.
'soldiers'' becomes 'RE' (Royal Engineers).
'mess' becomes 'hash' (making a hash of something is making a mess of it).
're'+'hash'='REHASH'
'showing' acts as a link.
Can you help me to learn more?
(Other definitions for rehash that I've seen before include "Use again, with few or no changes" , "Same material, different mix" , "new disposition" , "Reworking of old material" , "That's nothing really new" .)