Grim day to be on lifeboat? (4)
I believe the answer is:
dark
'grim' is the definition.
(synonyms)
'day to be on lifeboat?' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'to be on' means one lot of letters go next to another (in a down clue, letters appear on others).
'lifeboat?' becomes 'ark'.
'd'+'ark'='DARK'
(Other definitions for dark that I've seen before include "Overcast" , "Devoid of light" , "Nearer black than white" , "Secret, shadowed" , "Swarthy" .)