Heard collier was unimportant (5)
I believe the answer is:
minor
'unimportant' is the definition.
(I've seen this in another clue)
'heard collier' is the wordplay.
'heard' shows a homophone (sound like).
'collier' becomes 'miner' (I've seen this before).
'miner' sounds like 'MINOR'.
'was' acts as a link.
(Other definitions for minor that I've seen before include "Less important" , "Not major" , "Of little significance" , "Deceptively realistic (art)" , "Of small importance" .)