That sought by certain modern missiles? (4)

I believe the answer is:

heat

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'that sought by certain modern missiles?' is the definition.
I can't tell whether this definition defines the answer.

'that sought by' is the wordplay.
'that sought' becomes 'he' (I can't explain this - if you can you should believe this answer much more).
'by' becomes 'at'.
'he'+'at'='HEAT'

Can you help me to learn more?

(Other definitions for heat that I've seen before include "Warm up" , "Warmth - gossip mag" , "High temperature" , "Warmth - passion" , "Cook" .)

I've seen this clue in the Evening Standard.

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