A foot preserved in vitro - cheek! (7)
I believe the answer is:
trochee
'a foot' is the definition.
(trochee is a kind of foot)
'preserved in vitro cheek' is the wordplay.
'preserved in' says the answer is hidden in the clue.
'TROCHEE' is hidden in the letters of 'vitro cheek'.
(Other definitions for trochee that I've seen before include "A foot of two syllables, long then short" , "poetic feature" , "To cheer about this metrical unit" , "Long-short foot in poetry" , "Long-short metrical foot" .)