Heard collier was a child (5)
I believe the answer is:
minor
'a child' is the definition.
(I know that child can be written as minor)
'heard collier' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'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 "Paltry" , "Of little significance" , "Adolescent" , "Like clubs" , "Slight; little known" .)