Gathering coal is complicated (6)
I believe the answer is:
social
'gathering' is the definition.
(social is a kind of gathering)
'coal is complicated' is the wordplay.
'complicated' indicates anagramming the letters (a complex or confusing ordering of the letters).
'coal'+'is'='coalis'
'coalis' with letters rearranged gives 'SOCIAL'.
(Other definitions for social that I've seen before include "Convivial - party" , "Living in an organised community in Licosa" , "Pertaining to human relations" , "Gregarious or neighbourly" , "Informal gathering" .)