Gathering coal is messy (6)
I believe the answer is:
social
'gathering' is the definition.
(social is a kind of gathering)
'coal is messy' is the wordplay.
'messy' indicates anagramming the letters.
'coal'+'is'='coalis'
'coalis' is an anagram of 'SOCIAL'.
(Other definitions for social that I've seen before include "Informal gathering" , "Relating to human groups" , "Gregarious or neighbourly" , "Needing companionship" , "Of people; gathering" .)