Engineers fathom echo (7)
I believe the answer is:
resound
'echo' is the definition.
('resound' can be a synonym of 'echo')
'engineers fathom' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'fathom' becomes 'sound' ('sound' can be a synonym of 'fathom').
're'+'sound'='RESOUND'
(Other definitions for resound that I've seen before include "Reverberate" , "Echo" , "ring" .)