Warm up expending energy — control bowler's rest? (6)
I believe the answer is:
hatpeg
'bowler's rest?' is the definition.
(bowler is a type of hat)
'warm up expending energy control' is the wordplay.
'warm up' becomes 'heat' (I've seen this in another clue).
'expending' is a deletion indicator.
'energy' becomes 'e' (physics symbol as in E=mce).
'control' becomes 'peg' (peg is a kind of control).
'heat' with 'e' taken away is 'hat'.
'hat'+'peg'='HATPEG'
(Other definitions for hatpeg that I've seen before include "Where to hang cap, etc" , "where Kentucky Derby's held?" , "Cloakroom pin" , "fixture for bowler?" .)