Puzzle of engineers going by public transport (5)
I believe the answer is:
rebus
'puzzle' is the definition.
(I've seen this before)
'engineers going by public transport' is the wordplay.
'engineers' becomes 're' (Royal Engineers**).
'going by' means one lot of letters go next to another.
'public transport' becomes 'bus' (bus is a kind of public transport).
're'+'bus'='REBUS'
'of' acts as a link.
(Other definitions for rebus that I've seen before include "Picture-puzzle" , "Ian Rankin's Scottish detective" , "Pictorial wordplay" , "Visual wordplay - Rankin's inspector" , "Picture and word puzzle" .)