Tense when investing in source of coal power? (5)
I believe the answer is:
steam
'power?' is the definition.
(I've seen this before)
'tense when investing in source of coal' is the wordplay.
'tense' becomes 't' (abbreviation used in many dictionaries).
'when investing in' is an insertion indicator.
'source of coal' becomes 'seam' (as in a seam in a mine).
't' put inside 'seam' is 'STEAM'.
(Other definitions for steam that I've seen before include "Travel in liner" , "Source of power" , "Vapour from water at boiling temperature" , "Like old radio" , "Water-vapour" .)