Giving up iodine, lighten up for duration (6)
I believe the answer is:
length
'for duration' is the definition.
(synonyms)
'giving up iodine lighten up' is the wordplay.
'giving up' is a deletion indicator.
'iodine' becomes 'I' (I is the chemical symbol for iodine).
'up' is an anagram indicator (letters get chucked up).
'lighten' with 'i' taken away is 'lghten'.
'lghten' anagrammed gives 'LENGTH'.
(Other definitions for length that I've seen before include "Extent from end to end" , "Linear dimension" , "Measurement from end to end" , "A bit of swimming" , "A brief swim" .)