Body-builder in port, perhaps – a German one (7)
I believe the answer is:
protein
'body-builder' is the definition.
(I've seen this before)
'in port perhaps a german one' is the wordplay.
'in' is an insertion indicator.
'perhaps' indicates anagramming the letters.
'a german' becomes 'en' (I can't explain this - if you can you should give a lot more credence to this answer).
'one' becomes 'i' (Roman numeral).
'port' with letters rearranged gives 'prot'.
'prot'+'en'='proten'
'proten' placed around 'i' is 'PROTEIN'.
Can you help me to learn more?
(Other definitions for protein that I've seen before include "Organic compound, essential constituent of living cells" , "Substance in meat" , "Essential part of diet, found in meat, say" , "Combination of amino acids in living things" , "A body chemical; pointer (anag.)" .)