They suppress fire with engineers in southern dams (10)

'they suppress fire' is the definition.
(smotherers suppress or smother fire)

'engineers in southern dams' is the wordplay.
'engineers' becomes 're' (Royal Engineers).
'in' means one lot of letters goes inside another.
'southern' becomes 's' (abbreviation).
'dams' becomes 'mothers' (I've seen this before).
's'+'mothers'='smothers'
're' inserted within 'smothers' is 'SMOTHERERS'.

'with' acts as a link.