Wet and weak, is brought on board if that is desirable initially (9)

I believe the answer is:

sissified

I'm a little stuck... Click here to teach me more about this clue!

'wet and weak is brought on board if that is desirable initially' is the definition.
Both the answer and definition are adjectives. Perhaps there's an association between them I don't understand?

'board if that is desirable initially' is the wordplay.
'board' becomes 'siss' (I can't justify this - if you can you should believe this answer much more).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'initially' says to take the initial letters.
The first letter of 'desirable' is 'd'.
'siss'+'if'+'ie'+'d'='SISSIFIED'

Can you help me to learn more?

I've seen this clue in The Independent.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.