Crossword Genius

Heart monitor's first indicator (6)

Ross

I believe the answer is:

marrow

'heart' is the definition.
('marrow' can be a synonym of 'heart')

'monitor's first indicator' is the wordplay.
'first' suggests taking the first letters.
'indicator' becomes 'arrow' (I've seen this before).
The first letter of 'monitors' is 'm'.
'm'+'arrow'='MARROW'

(Other definitions for marrow that I've seen before include "The best part" , "Bone filler" , "Edible gourd" , "Vegetable - content of a bone" , "Fatty stuff in bones or large fruit" .)

I've seen this clue in The Telegraph.
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