Every so often, sponge grime or it will become more waterlogged (7)
I believe the answer is:
soggier
'more waterlogged' is the definition.
('soggy' can be a synonym of 'waterlogged')
'every so often sponge grime or' is the wordplay.
'every so often' indicates alternate letters.
'sponge'+'grime'+'or'='spongegrimeor'
The alternate letters of 'spongegrimeor' are 'SOGGIER'.
'it will become' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Another definition for soggier that I've seen is " wetter".)