Sea air adds a little weight to one (5)
I believe the answer is:
ozone
'sea air' is the definition.
(I've seen this before)
'weight to one' is the wordplay.
'weight' becomes 'oz'.
'oz'+'one'='OZONE'
'adds a little' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
(Other definitions for ozone that I've seen before include "It's said to brace" , "Layer that protects against ultraviolet rays" , "Bracing air of old, now seen as potentially toxic" , "Condensed oxygen; sea air" , "(Depleted) layer of gas" .)