Perhaps dredge river in front of submarine dock (6)
I believe the answer is:
deepen
'perhaps dredge' is the definition.
'deepen' can be an answer for 'dredge' (I've seen this in another clue). I am not certain of the 'perhaps' bit.
'river in front of submarine dock' is the wordplay.
'river' becomes 'dee' (River Dee).
'in front of' means one lot of letters go next to another (some letters go in front of others).
'submarine dock' becomes 'pen' (I've seen this in other clues. I am not sure about the 'submarine' bit.).
'dee'+'pen'='DEEPEN'
Can you help me to learn more?
(Other definitions for deepen that I've seen before include "To grow" , "Go lower" , "Intensify" , "make room for more water" , "Make (colour) darker" .)