Become visible now, in the gloom (6)
I believe the answer is:
shadow
'gloom' is the definition.
(I've seen this before)
'become visible now in' is the wordplay.
'become visible' becomes 'show' (I can't justify this - if you can you should give a lot more credence to this answer).
'now' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'in' indicates putting letters inside.
'show' placed around 'ad' is 'SHADOW'.
'the' acts as a link.
Can you help me to learn more?
(Other definitions for shadow that I've seen before include "Secretly tail someone" , "Follow - dark patch" , "This opposition cabinet parallels the government" , "Unilluminated area" , "Dark area - referring to parliamentary opposition" .)