Lament taking up online crossword? (5)
I believe the answer is:
dirge
'lament' is the definition.
(synonyms)
'up online crossword?' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'online' becomes 'e' (abbreviation for electronic, e.g. in ecommerce).
'crossword?' becomes 'grid' (a crossword grid).
'e'+'grid'='egrid'
'egrid' backwards is 'DIRGE'.
'taking' is the link.
(Other definitions for dirge that I've seen before include "Mournful chant" , "Funereal music" , "Funeral hymn" , "Mournful music" , "Dragging piece" .)