Engineers left one cold souvenir (5)
I believe the answer is:
relic
'souvenir' is the definition.
(I know that keepsake can be written as relic)
'engineers left one cold' is the wordplay.
'engineers' becomes 're' (abbreviation for Royal Engineers).
'left' becomes 'l' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'cold' becomes 'c' (eg on taps).
're'+'l'+'i'+'c'='RELIC'
(Other definitions for relic that I've seen before include "Memorial of antiquity" , "Paper craft" , "Object that has survived from the past" , "Surviving ancient custom" , "Memento; souvenir" .)