Aquatic creature almost always next to lake causes rising concern (3,5)
I believe the answer is:
sea level
'rising concern' is the definition.
(rising sea levels are a concern)
'aquatic creature almost always next to lake' is the wordplay.
'aquatic creature' becomes 'seal'.
'almost' means to remove the last letter (most of the word but not all of it).
'always' becomes 'ever' (synonyms).
'next to' means one lot of letters go next to another.
'lake' becomes 'l'.
'ever' with its final letter taken off is 'eve'.
'seal'+'eve'+'l'='SEA-LEVEL'
'causes' acts as a link.
(Other definitions for sea level that I've seen before include "Base from which height of aircraft is measured" , "Standard for measuring map heights" , "0 feet?" , "From which the heights of hills etc are measured" , "It's used in reckoning land elevation" .)