Engineers going by river and sea got back (8)
I believe the answer is:
redeemed
'got back' is the definition.
(I've seen this in another clue)
'engineers going by river and sea' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'going by' says to put letters next to each other.
'river' becomes 'dee' (River Dee).
'and' says to put letters next to each other.
'sea' becomes 'med' (short for Mediterranean).
're'+'dee'+'med'='REDEEMED'
(Other definitions for redeemed that I've seen before include "Thought about the possibilities again" , "Recovered possession - paid back" , "bought back from uncle?" , "exchanged tokens" , "Atoned for" .)