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' says to put letters next to each other (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" .)