Certain letters used partially as a means of entry (5)
I believe the answer is:
inlet
'a means of entry' is the definition.
(I've seen this before)
'certain letters used partially' is the wordplay.
'partially' says the answer is hidden in the clue.
'INLET' can be found hidden inside 'certain letters used'.
'as' acts as a link.
(Other definitions for inlet that I've seen before include "Entry; arm of the sea" , "Entry port" , "Fiord" , "Strip of water extending from sea into land" , "Small arm of the sea" .)