Elastic pressure in fluctuating pulse (6)
I believe the answer is:
supple
'elastic' is the definition.
(I've seen this in another clue)
'pressure in fluctuating pulse' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'in' means one lot of letters goes inside another.
'fluctuating' is an anagram indicator.
'pulse' anagrammed gives 'suple'.
'p' going within 'suple' is 'SUPPLE'.
(Other definitions for supple that I've seen before include "Mobile" , "Lithe, pliant" , "Lissom" , "Lithe and flexible" , "Flexible (body)" .)