Tenacious types have their first busy year on board (7)

'tenacious types' is the definition.
Although both the answer and definition are plural nouns, I can't see how one could define the other.

'their first busy year on board' is the wordplay.
'their first' becomes 'sts' (I can't justify this - if you can you should give a lot more credence to this answer).
'busy' indicates anagramming the letters.
'on board' is an insertion indicator (inserted letters are taken on board).
'year' with letters rearranged gives 'ayer'.
'sts' going around 'ayer' is 'STAYERS'.

'have' acts as a link.

Can you help me to learn more?