Not the main one tendering for underwater craft?
I believe the answer is:
subcontractor
'one tendering for underwater craft?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'not the main' is the wordplay.
I cannot really see how this works, but
an anagram of 'not' is 'ont' which is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for subcontractor that I've seen is " Firm undertaking part of another's agreement".)