Writer crossing key expanse of water (6)
I believe the answer is:
baltic
'expanse of water' is the definition.
'baltic' can be an answer for 'water' (Baltic is an example). I am not certain of the 'expanse of' bit.
'writer crossing key' is the wordplay.
'writer' becomes 'bic' (kind of pen).
'crossing' indicates putting letters inside (to cross can mean to go over or around).
'key' becomes 'alt' (the Alt keyboard button).
'bic' enclosing 'alt' is 'BALTIC'.
Can you help me to learn more?
(Other definitions for baltic that I've seen before include "Almost land-locked sea of Scandinavia" , "Sea of northern Europe" , "Northern European sea" , "cold waters" , "North European sea" .)