Screen nitrogen with it in heathland (7)
I believe the answer is:
monitor
'screen' is the definition.
(computer screen)
'nitrogen with it in heathland' is the wordplay.
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'with' says to put letters next to each other.
'in' indicates putting letters inside.
'heathland' becomes 'moor' (I've seen this before).
'n'+'it'='nit'
'nit' placed into 'moor' is 'MONITOR'.
(Other definitions for monitor that I've seen before include "Keep a check on" , "what a computer user may require" , "Lizard; display screen" , "Person or device for observing and checking" , "record" .)