Factory temperature is below target (5)
I believe the answer is:
plant
'factory' is the definition.
(I've seen this before)
'temperature is below target' is the wordplay.
'temperature' becomes 't' (abbreviation).
'is below' says to put letters next to each other (in a down clue, some letters go below others).
'target' becomes 'plan'.
't' put after 'plan' is 'PLANT'.
(Other definitions for plant that I've seen before include "Machinery; snooker shot" , "that goes to pot" , "Put in the ground to grow" , "Put in soil" , "Factory; informer" .)