Engineers attending a rip behind (4,3)
I believe the answer is:
rear end
'behind' is the definition.
(synonyms)
'engineers attending a rip' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'attending' is a charade indicator (letters next to each other).
'rip' becomes 'rend' (synonyms).
're'+'a'+'rend'='REAR-END'
(Other definitions for rear end that I've seen before include "run into the car in front" , "Backside (informal)" , "Run into another vehicle from behind" , "Buttocks (informal)" , "The buttocks (colloq.)" .)