Engineers keep it as an observance (4)
I believe the answer is:
rite
'an observance' is the definition.
(thesaurus)
'engineers keep it' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'keep' indicates putting letters inside.
're' placed around 'it' is 'RITE'.
'as' is the link.
(Other definitions for rite that I've seen before include "Customary observance" , "form of service" , "Ceremonial procedure" , "Religious ritual" , "Tire of ceremony" .)