Left back greeting captain concerned with nutrition (7)
I believe the answer is:
trophic
'captain concerned with nutrition' is the definition.
I can't judge whether this definition defines the answer.
'left back greeting' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'back' is a reversal indicator.
'greeting' becomes 'hic' (I can't justify this - if you can you should believe this answer much more).
'port' backwards is 'trop'.
'trop'+'hic'='TROPHIC'
Can you help me to learn more?
(Other definitions for trophic that I've seen before include "as nutrition" , "of food" .)