Coaler is giving out units of heat (8)
I believe the answer is:
calories
'units of heat' is the definition.
(I've seen this before)
'coaler is giving' is the wordplay.
'giving' indicates an anagram (the letters give the letters for another word).
'coaler'+'is'='coaleris'
'coaleris' is an anagram of 'CALORIES'.
'out' acts as a link.
This may not be correct. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for calories that I've seen before include "Energy or heat units from Escorial" , "Units of food value from Escorial" , "Measures for the energy value of food" , "Units of energy in food" , "Units of heat, counted on diets" .)