Reluctant to assert the limits of science (6)
I believe the answer is:
averse
'reluctant' is the definition.
(thesaurus)
'assert the limits of science' is the wordplay.
'assert' becomes 'aver' (averring is a kind of asserting).
'the limits of' suggests removing the centre.
'science' with its centre taken out is 'se'.
'aver'+'se'='AVERSE'
'to' is the link.
(Other definitions for averse that I've seen before include "A piece of poetry is reluctant" , "Metal rod; card game" , "Disinclined, loath" , "Disinclined, maybe to hard work" , "Strongly opposed (to something)" .)