Give reasons why — in centrepiece of text, straightforward (7)
I believe the answer is:
'give reasons why' is the definition.
(to explain something is to give reasons for it)
'centrepiece of text straightforward' is the wordplay.
'centrepiece of' means to look at the middle letters (piece from the middle).
'straightforward' becomes 'plain' (I've seen this before).
The central letters of 'text' are 'ex'.
'in' acts as a link.
(Other definitions for explain that I've seen before include "offer clarification" , "Shed light" , "area affected by mountain formation?" , "Resolve" , "Interpret" .)