Pieces, all the same length, needed for decongestant (7)
I believe the answer is:
menthol
'decongestant' is the definition.
'pieces all the same length needed' is the wordplay.
'pieces' becomes 'men' ('man' can be a synonym of 'piece'**).
'all the same' becomes 'tho' (I've seen this before).
'length' becomes 'l'.
'needed' means one lot of letters go next to another.
'men'+'tho'+'l'='MENTHOL'
'for' is the link.
(Other definitions for menthol that I've seen before include "Soothing substance from peppermint" , "Peppermint camphor" , "Chemical compound that smells like mint" , "Camphor used as a decongestant" , "mild analgesic" .)