Royal Engineers gathering in memory (12)
I believe the answer is:
recollection
'memory' is the definition.
(recollection is a kind of memory)
'royal engineers gathering' is the wordplay.
'royal engineers' becomes 're'.
'gathering' becomes 'collection' (gathering is collecting things together).
're'+'collection'='RECOLLECTION'
'in' is the link.
(Other definitions for recollection that I've seen before include "Memory" , "mental image" , "Remembrance" , "Recall" .)