Lines written in blank diary inclined to drop off (6)
I believe the answer is:
drowsy
'inclined to drop off' is the definition.
(I know that drowsy is the attribute pertaining to somnolence)
'lines written in blank diary' is the wordplay.
'lines' becomes 'rows' (row is a kind of line).
'written in' means one lot of letters goes inside another.
'blank' suggests removing the centre (the word is emptied out or 'blank').
'diary' with its middle removed is 'dy'.
'rows' going inside 'dy' is 'DROWSY'.
(Other definitions for drowsy that I've seen before include "Wanting to nod off" , "Inclined to nod off" , "Sleepy, half-asleep" , "Almost asleep" , "Half asleep" .)