Johnson's occasionally in blue during visit (7)
I believe the answer is:
stanley
'johnson's' is the definition.
(Stanley Johnson, father of Boris Johnson)
'occasionally in blue during visit' is the wordplay.
'occasionally' means one should take alternating letters.
'during' is an insertion indicator.
'visit' becomes 'stay' (I've seen this before).
'in'+'blue'='inblue'
The alternate letters of 'inblue' are 'nle'.
'nle' going inside 'stay' is 'STANLEY'.
(Other definitions for stanley that I've seen before include "African explorer; Falklands town" , "He found Livingstone" , "Type of knife" , "He said 'Dr Livingstone, I presume'" , "Falklands capital; --- Baldwin, PM" .)