Weak drink I kept in mind, when heading off (7)
I believe the answer is:
insipid
'weak' is the definition.
(thesaurus)
'drink i kept in mind when heading off' is the wordplay.
'drink' becomes 'sip' (sipping is a kind of drinking).
'kept in' indicates putting letters inside.
'when heading off' means to remove the first letter.
'sip'+'i'='sipi'
'mind' with its first letter taken away is 'ind'.
'sipi' put into 'ind' is 'INSIPID'.
(Other definitions for insipid that I've seen before include "Lacking in flavour or interest" , "Lacking taste or flavour" , "Bland, flavourless" , "Dull" , "Tasteless" .)