Impassive fire controller, female, provided great service (5-5)
I believe the answer is:
poker-faced
'impassive' is the definition.
(I know that impassive can be written as poker-faced)
'fire controller female provided great service' is the wordplay.
I cannot really understand how this works, but
'female' could be 'fa' (I've seen this in other clues) and 'fa' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for poker-faced that I've seen before include "Giving nothing away?" , "perfect air for 23 [STUD]" .)