Gradually develop passion for climbing: very English! (6)
I believe the answer is:
evolve
'gradually develop' is the definition.
(I've seen this before)
'passion for climbing very english' is the wordplay.
'passion' becomes 'love' ('love' can be a synonym of 'passion'**).
'for climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'very' becomes 'v' (abbreviation).
'english' becomes 'e' (abbreviation).
'love' back-to-front is 'evol'.
'evol'+'v'+'e'='EVOLVE'
(Other definitions for evolve that I've seen before include "Develop slowly" , "Unfold" , "Change gradually to present condition" , "Cartoon sailor" , "Develop or change very gradually (6)" .)