Part of pipe introducing a water source (5)
I believe the answer is:
steam
'water source' is the definition.
(steam is water vapour)
'part of pipe introducing a' is the wordplay.
'part of pipe' becomes 'stem' ('stem' is part of 'pipe').
'introducing' indicates putting letters inside (some letters introduced or inserted into others).
'stem' enclosing 'a' is 'STEAM'.
(Other definitions for steam that I've seen before include "Water vapour; impetus" , "Water-vapour" , "Watery vapour" , "Old-fashioned power" , "Vapour from water at boiling point" .)