Idle talk about head of engineering plant (4)
I believe the answer is:
sage
'plant' is the definition.
(sage is a kind of plant)
'idle talk about head of engineering' is the wordplay.
'idle talk' becomes 'gas' (I've seen this before).
'about' is a reversal indicator.
'head of' suggests taking the first letters.
The first letter of 'engineering' is 'e'.
'gas' in reverse letter order is 'sag'.
'sag'+'e'='SAGE'
(Other definitions for sage that I've seen before include "Fragrant herb having wisdom" , "Profoundly wise" , "Herb used in cooking" , "Oracle; herb" , "Herb for the wise one" .)