An element still keeping chin up, periodically (7)
I believe the answer is:
calcium
'an element' is the definition.
(calcium is a kind of element)
'still keeping chin up periodically' is the wordplay.
'still' becomes 'calm' ('calm' can be a synonym of 'still'**).
'keeping' indicates putting letters inside.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'chin'+'up'='chinup'
The alternate letters of 'chinup' are 'ciu'.
'calm' enclosing 'ciu' is 'CALCIUM'.
(Other definitions for calcium that I've seen before include "Bone constituent" , "Essential constituent of bones and teeth" , "White metallic element found in most animals and plants, Ca" , "Metallic element present in limestone" , "White metallic element in limestone, chalk" .)