U-boats with bronze and titanium from war partner, mainly (13)

I believe the answer is:

substantially

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

'mainly' is the definition.
(thesaurus)

'u-boats with bronze and titanium from war partner' is the wordplay.
I cannot quite see how this works, but
'u' is present in the answer.
'bronze' could be 'tan' (both can mean to get a sun tan) and 'tan' is located in the answer.
'titanium' could be 'ti' (Ti is the chemical symbol for titanium) and 'ti' is located in the answer.
'partner' could be 'ally' (I've seen this in another clue) and 'ally' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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.