themer

One who themes something.

One who themes something.

This structure organizes and streamlines theme development to create a solid ground for themers (theme developers) to further build and extend, much like in object-oriented programming.

An entry in a crossword that forms part of the theme.

The first of the themers is SPARRING PARTNER (“Opponent who helps train a boxer”) — which, as the revealer indicates, includes the word PAR twice.

I threw in a bonus with ASTHMA – Longest familiar word in the English language that begins and ends with a vowel with no vowels in between (though you may not want to use that clue since there is no symmetrical themer).