A father secures power to rise to a great height (6)
I believe the answer is:
aspire
'rise to a great height' is the definition.
(I've seen this before)
'a father secures power' is the wordplay.
'father' becomes 'sire' (synonyms).
'secures' means one lot of letters goes inside another.
'power' becomes 'p' (symbol in physics).
'sire' going around 'p' is 'spire'.
'a'+'spire'='ASPIRE'
'to' acts as a link.
(Other definitions for aspire that I've seen before include "Aim (for higher things)" , "Hope (to achieve)" , "Hanker" , "Desire (to reach higher things)" , "Yearn, dream (towards)" .)