Top deck on old ship is lacking lights (4)
I believe the answer is:
dark
'lacking lights' is the definition.
(I've seen this before)
'top deck on old ship' is the wordplay.
'top' says to take the initial letters.
'on old ship' becomes 'ark' (I can't justify this - if you can you should give a lot more credence to this answer).
The initial letter of 'deck' is 'd'.
'd'+'ark'='DARK'
'is' is the link.
Can you help me to learn more?
(Other definitions for dark that I've seen before include "lacking lights" , "Swarthy" , "Opposite of light" , "With little or no light" , "Shade of Oxford blue" .)