Engineers assembled first at a certain distance (5)
I believe the answer is:
metre
'a certain distance' is the definition.
'metre' can be an answer for 'distance' (unit of distance). I'm unsure of the 'a certain' bit.
'engineers assembled first' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'assembled' becomes 'met' ('meet' can be a synonym of 'assemble').
'first' says to put letters next to each other.
're' after 'met' is 'METRE'.
'at' is the link.
Can you help me to learn more?
(Other definitions for metre that I've seen before include "Approx 39 inches" , "Basic unit of length" , "100cm" , "Distance" , "Characteristic of feet" .)