Clue: Theologian and Darwin swapping tips for theorem (7)
Answer: ERASTUS
Explanation: Theologian and Darwin swapping tips for theorem (7)
**ERAS**(m)(**T**)**US **(Erasmus "Darwin" with one tip of "theorem" (M) swapped for the other tip (T)) 
*A fair amount of general knowledge required here, I'd suggest, as neither Erastus, not Erasmus Darwin, Charles's grandfather, are that well known.