Delta stops fish reaching a lake (5)
I believe the answer is:
garda
'lake' is the definition.
(I've seen this before)
'delta stops fish reaching a' is the wordplay.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'stops' indicates putting letters inside.
'fish' becomes 'gar' (gar is a kind of fish).
'reaching' means one lot of letters go next to another.
'gar'+'a'='gara'
'd' inserted within 'gara' is 'GARDA'.
(Other definitions for garda that I've seen before include "Cop" , "One of our police - what a drag" , "Italian lake - Irish policeman" , "Irish police officer" , "Irish state police force" .)