Gather nearly all, but every second one is hellish (8)
I believe the answer is:
infernal
'one is hellish' is the definition.
(synonyms)
'gather nearly all but every second' is the wordplay.
'gather' becomes 'infer' (synonyms).
'nearly all but every second' becomes 'nal' (I can't justify this - if you can you should give a lot more credence to this answer).
'infer'+'nal'='INFERNAL'
Can you help me to learn more?
(Other definitions for infernal that I've seen before include "Extremely annoying" , "To do with hell, or informally detestable" , "Relating to the underworld" , "evil here" , "Of hell, or tiresome like a nuisance" .)