Idly read lines in bed, mostly (6)
I believe the answer is:
browse
'idly read' is the definition.
(to browse is to read in an idle way)
'lines in bed mostly' is the wordplay.
'lines' becomes 'rows' (row is a kind of line).
'in' means one lot of letters goes inside another.
'mostly' means to remove the last letter (most but not all of the word).
'bed' with its final letter removed is 'be'.
'rows' inserted inside 'be' is 'BROWSE'.
(Other definitions for browse that I've seen before include "Surf (net)" , "Read in a bookshop - graze" , "Leaf through (a book)" , "Read idly" , "Read material online" .)