Sun helmet is cretonne in centre? It’s irrational! (4)
I believe the answer is:
topi
'sun helmet' is the definition.
(I know that sun helmet can be written as topi)
'cretonne in centre? it's irrational' is the wordplay.
'in centre?' means to look at the middle letters.
'it's irrational' becomes 'pi' (an irrational number).
The central letters of 'cretonne' are 'to'.
'to'+'pi'='TOPI'
'is' is the link.
(Other definitions for topi that I've seen before include "African antelope" , "Pith helmet" , "Tropical sun-hat" , "cover in the heat" , "Sun protection" .)