The most common and most likely answer to this crossword clue is the 7 letter word COOLANT
I've seen this crossword clue in Independent
C
O
O
L
A
N
T
Buddy explains!
The straight part
We know that "part of heat exchanger" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of heat exchanger = coolant .The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does COOLANT mean?
COOLANT noun- A medium, usually fluid, used to draw heat from an object.