Old craft by day - or night (4)
I believe the answer is:
dark
'night' is the definition.
(synonyms)
'old craft by day' is the wordplay.
'old craft' becomes 'ark' (ark is a kind of craft. I am not sure about the 'old' bit.).
'by' says to put letters next to each other.
'day' becomes 'd' (abbreviation for day).
'ark' put after 'd' is 'DARK'.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for dark that I've seen before include "Nearer black than white" , "Sombre in colour" , "Nightfall; without light" , "lacking lights" , "Such an unknown horse" .)